diff --git a/src/Squidex.Domain.Apps.Core.Model/Schemas/AssetsFieldProperties.cs b/src/Squidex.Domain.Apps.Core.Model/Schemas/AssetsFieldProperties.cs
index 49e4d3c63..7566e6864 100644
--- a/src/Squidex.Domain.Apps.Core.Model/Schemas/AssetsFieldProperties.cs
+++ b/src/Squidex.Domain.Apps.Core.Model/Schemas/AssetsFieldProperties.cs
@@ -33,6 +33,8 @@ namespace Squidex.Domain.Apps.Core.Schemas
 
         public int? AspectHeight { get; set; }
 
+        public bool AllowDuplicates { get; set; }
+
         public ReadOnlyCollection<string> AllowedExtensions { get; set; }
 
         public override T Accept<T>(IFieldPropertiesVisitor<T> visitor)
diff --git a/src/Squidex.Domain.Apps.Core.Model/Schemas/ReferencesFieldProperties.cs b/src/Squidex.Domain.Apps.Core.Model/Schemas/ReferencesFieldProperties.cs
index c6af16a90..632e5b9bf 100644
--- a/src/Squidex.Domain.Apps.Core.Model/Schemas/ReferencesFieldProperties.cs
+++ b/src/Squidex.Domain.Apps.Core.Model/Schemas/ReferencesFieldProperties.cs
@@ -15,6 +15,8 @@ namespace Squidex.Domain.Apps.Core.Schemas
 
         public int? MaxItems { get; set; }
 
+        public bool AllowDuplicates { get; set; }
+
         public Guid SchemaId { get; set; }
 
         public override T Accept<T>(IFieldPropertiesVisitor<T> visitor)
diff --git a/src/Squidex.Domain.Apps.Core.Operations/ValidateContent/ValidatorsFactory.cs b/src/Squidex.Domain.Apps.Core.Operations/ValidateContent/ValidatorsFactory.cs
index af7e3e698..ae6351a91 100644
--- a/src/Squidex.Domain.Apps.Core.Operations/ValidateContent/ValidatorsFactory.cs
+++ b/src/Squidex.Domain.Apps.Core.Operations/ValidateContent/ValidatorsFactory.cs
@@ -55,6 +55,11 @@ namespace Squidex.Domain.Apps.Core.ValidateContent
                 yield return new CollectionValidator(field.Properties.IsRequired, field.Properties.MinItems, field.Properties.MaxItems);
             }
 
+            if (!field.Properties.AllowDuplicates)
+            {
+                yield return new UniqueValuesValidator<Guid>();
+            }
+
             yield return new AssetsValidator(field.Properties);
         }
 
@@ -125,6 +130,11 @@ namespace Squidex.Domain.Apps.Core.ValidateContent
                 yield return new CollectionValidator(field.Properties.IsRequired, field.Properties.MinItems, field.Properties.MaxItems);
             }
 
+            if (!field.Properties.AllowDuplicates)
+            {
+                yield return new UniqueValuesValidator<Guid>();
+            }
+
             if (field.Properties.SchemaId != Guid.Empty)
             {
                 yield return new ReferencesValidator(field.Properties.SchemaId);
diff --git a/src/Squidex/Areas/Api/Controllers/Schemas/Models/Fields/AssetsFieldPropertiesDto.cs b/src/Squidex/Areas/Api/Controllers/Schemas/Models/Fields/AssetsFieldPropertiesDto.cs
index 30a43fd08..9ef6cff0c 100644
--- a/src/Squidex/Areas/Api/Controllers/Schemas/Models/Fields/AssetsFieldPropertiesDto.cs
+++ b/src/Squidex/Areas/Api/Controllers/Schemas/Models/Fields/AssetsFieldPropertiesDto.cs
@@ -73,6 +73,11 @@ namespace Squidex.Areas.Api.Controllers.Schemas.Models.Fields
         /// </summary>
         public string[] AllowedExtensions { get; set; }
 
+        /// <summary>
+        /// True, if duplicate values are allowed.
+        /// </summary>
+        public bool AllowDuplicates { get; set; }
+
         public override FieldProperties ToProperties()
         {
             var result = SimpleMapper.Map(this, new AssetsFieldProperties());
diff --git a/src/Squidex/Areas/Api/Controllers/Schemas/Models/Fields/ReferencesFieldPropertiesDto.cs b/src/Squidex/Areas/Api/Controllers/Schemas/Models/Fields/ReferencesFieldPropertiesDto.cs
index 07ee52dc5..8e73ed212 100644
--- a/src/Squidex/Areas/Api/Controllers/Schemas/Models/Fields/ReferencesFieldPropertiesDto.cs
+++ b/src/Squidex/Areas/Api/Controllers/Schemas/Models/Fields/ReferencesFieldPropertiesDto.cs
@@ -23,6 +23,11 @@ namespace Squidex.Areas.Api.Controllers.Schemas.Models.Fields
         /// </summary>
         public int? MaxItems { get; set; }
 
+        /// <summary>
+        /// True, if duplicate values are allowed.
+        /// </summary>
+        public bool AllowDuplicates { get; set; }
+
         /// <summary>
         /// The id of the referenced schema.
         /// </summary>
diff --git a/src/Squidex/app/features/schemas/pages/schema/types/assets-validation.component.html b/src/Squidex/app/features/schemas/pages/schema/types/assets-validation.component.html
index 722b4b726..98c89760d 100644
--- a/src/Squidex/app/features/schemas/pages/schema/types/assets-validation.component.html
+++ b/src/Squidex/app/features/schemas/pages/schema/types/assets-validation.component.html
@@ -94,6 +94,17 @@
             <label class="col-form-label">px</label>
         </div>
     </div>
+
+    <div class="form-group row">
+        <div class="col-9 offset-3">
+            <div class="form-check">
+                <input class="form-check-input" type="checkbox" id="{{field.fieldId}}_fieldAllowDuplicates" formControlName="allowDuplicates" />
+                <label class="form-check-label" for="{{field.fieldId}}_fieldAllowDuplicates">
+                    Allow duplicate values
+                </label>
+            </div>
+        </div>
+    </div>
     
     <div class="form-group2 row">
         <label class="col-3 col-form-label">
diff --git a/src/Squidex/app/features/schemas/pages/schema/types/assets-validation.component.ts b/src/Squidex/app/features/schemas/pages/schema/types/assets-validation.component.ts
index 992ddba3b..811ce0351 100644
--- a/src/Squidex/app/features/schemas/pages/schema/types/assets-validation.component.ts
+++ b/src/Squidex/app/features/schemas/pages/schema/types/assets-validation.component.ts
@@ -61,5 +61,8 @@ export class AssetsValidationComponent implements OnInit {
 
         this.editForm.setControl('aspectHeight',
             new FormControl(this.properties.aspectHeight));
+
+        this.editForm.setControl('allowDuplicates',
+            new FormControl(this.properties.allowDuplicates));
     }
 }
\ No newline at end of file
diff --git a/src/Squidex/app/features/schemas/pages/schema/types/references-validation.component.html b/src/Squidex/app/features/schemas/pages/schema/types/references-validation.component.html
index cac8256c3..1f1da83bd 100644
--- a/src/Squidex/app/features/schemas/pages/schema/types/references-validation.component.html
+++ b/src/Squidex/app/features/schemas/pages/schema/types/references-validation.component.html
@@ -21,6 +21,17 @@
         </div>
     </div>
 
+    <div class="form-group row">
+        <div class="col-9 offset-3">
+            <div class="form-check">
+                <input class="form-check-input" type="checkbox" id="{{field.fieldId}}_fieldAllowDuplicates" formControlName="allowDuplicates" />
+                <label class="form-check-label" for="{{field.fieldId}}_fieldAllowDuplicates">
+                    Allow duplicate values
+                </label>
+            </div>
+        </div>
+    </div>
+
     <div class="form-group row">
         <label class="col-3 col-form-label">Items</label>
 
diff --git a/src/Squidex/app/features/schemas/pages/schema/types/references-validation.component.ts b/src/Squidex/app/features/schemas/pages/schema/types/references-validation.component.ts
index a7fcc797f..013a5141e 100644
--- a/src/Squidex/app/features/schemas/pages/schema/types/references-validation.component.ts
+++ b/src/Squidex/app/features/schemas/pages/schema/types/references-validation.component.ts
@@ -31,6 +31,9 @@ export class ReferencesValidationComponent implements OnInit {
     }
 
     public ngOnInit() {
+        this.editForm.setControl('allowDuplicates',
+            new FormControl(this.properties.allowDuplicates));
+
         this.editForm.setControl('maxItems',
             new FormControl(this.properties.maxItems));
 
diff --git a/src/Squidex/app/shared/services/schemas.types.ts b/src/Squidex/app/shared/services/schemas.types.ts
index ba94c6567..1da787806 100644
--- a/src/Squidex/app/shared/services/schemas.types.ts
+++ b/src/Squidex/app/shared/services/schemas.types.ts
@@ -169,6 +169,7 @@ export class AssetsFieldPropertiesDto extends FieldPropertiesDto {
     public readonly maxHeight?: number;
     public readonly aspectWidth?: number;
     public readonly aspectHeight?: number;
+    public readonly allowDuplicates?: boolean;
 
     public get isSortable() {
         return false;
@@ -296,6 +297,7 @@ export class ReferencesFieldPropertiesDto extends FieldPropertiesDto {
     public readonly minItems?: number;
     public readonly maxItems?: number;
     public readonly schemaId?: string;
+    public readonly allowDuplicates?: boolean;
 
     public get isSortable() {
         return false;
diff --git a/src/Squidex/app/shared/state/contents.forms.spec.ts b/src/Squidex/app/shared/state/contents.forms.spec.ts
index 7f7d11b04..bf7424bf0 100644
--- a/src/Squidex/app/shared/state/contents.forms.spec.ts
+++ b/src/Squidex/app/shared/state/contents.forms.spec.ts
@@ -145,7 +145,7 @@ describe('AssetsField', () => {
     const field = createField(new AssetsFieldPropertiesDto({ isRequired: true, minItems: 1, maxItems: 5 }));
 
     it('should create validators', () => {
-        expect(FieldValidatorsFactory.createValidators(field, false).length).toBe(2);
+        expect(FieldValidatorsFactory.createValidators(field, false).length).toBe(3);
     });
 
     it('should format to empty string if null', () => {
@@ -328,7 +328,7 @@ describe('ReferencesField', () => {
     const field = createField(new ReferencesFieldPropertiesDto({ isRequired: true, minItems: 1, maxItems: 5 }));
 
     it('should create validators', () => {
-        expect(FieldValidatorsFactory.createValidators(field, false).length).toBe(2);
+        expect(FieldValidatorsFactory.createValidators(field, false).length).toBe(3);
     });
 
     it('should format to empty string if null', () => {
diff --git a/src/Squidex/app/shared/state/contents.forms.ts b/src/Squidex/app/shared/state/contents.forms.ts
index 20c07a9bf..f63e51c53 100644
--- a/src/Squidex/app/shared/state/contents.forms.ts
+++ b/src/Squidex/app/shared/state/contents.forms.ts
@@ -197,6 +197,10 @@ export class FieldValidatorsFactory implements FieldPropertiesVisitor<ValidatorF
             ValidatorsEx.betweenLength(properties.minItems, properties.maxItems)
         ];
 
+        if (!properties.allowDuplicates) {
+            validators.push(ValidatorsEx.uniqueStrings());
+        }
+
         return validators;
     }
 
@@ -205,6 +209,10 @@ export class FieldValidatorsFactory implements FieldPropertiesVisitor<ValidatorF
             ValidatorsEx.betweenLength(properties.minItems, properties.maxItems)
         ];
 
+        if (!properties.allowDuplicates) {
+            validators.push(ValidatorsEx.uniqueStrings());
+        }
+
         return validators;
     }
 
diff --git a/tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/AssetsFieldTests.cs b/tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/AssetsFieldTests.cs
index b0cd01ea5..38497a7af 100644
--- a/tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/AssetsFieldTests.cs
+++ b/tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/AssetsFieldTests.cs
@@ -49,7 +49,17 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
             PixelHeight = null
         };
 
-        private readonly AssetInfo image = new AssetInfo
+        private readonly AssetInfo image1 = new AssetInfo
+        {
+            AssetId = Guid.NewGuid(),
+            FileName = "MyImage.png",
+            FileSize = 1024 * 8,
+            IsImage = true,
+            PixelWidth = 800,
+            PixelHeight = 600
+        };
+
+        private readonly AssetInfo image2 = new AssetInfo
         {
             AssetId = Guid.NewGuid(),
             FileName = "MyImage.png",
@@ -63,7 +73,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
 
         public AssetsFieldTests()
         {
-            ctx = ValidationTestExtensions.Assets(image, document);
+            ctx = ValidationTestExtensions.Assets(image1, image2, document);
         }
 
         [Fact]
@@ -99,7 +109,17 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { MinItems = 2, MaxItems = 2 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, document.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(image1.AssetId, image2.AssetId), errors, ctx);
+
+            Assert.Empty(errors);
+        }
+
+        [Fact]
+        public async Task Should_not_add_error_if_duplicate_values_are_ignored()
+        {
+            var sut = Field(new AssetsFieldProperties { AllowDuplicates = true });
+
+            await sut.ValidateAsync(CreateValue(image1.AssetId, image1.AssetId), errors, ctx);
 
             Assert.Empty(errors);
         }
@@ -142,7 +162,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { MinItems = 3 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, document.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(image1.AssetId, image2.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "Must have at least 3 item(s)." });
@@ -153,7 +173,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { MaxItems = 1 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, document.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(image1.AssetId, image2.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "Must not have more than 1 item(s)." });
@@ -177,7 +197,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { MinSize = 5 * 1024 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, image.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(document.AssetId, image1.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "[1]: \'4 kB\' less than minimum of \'5 kB\'." });
@@ -188,7 +208,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { MaxSize = 5 * 1024 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, image.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(document.AssetId, image1.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "[2]: \'8 kB\' greater than maximum of \'5 kB\'." });
@@ -199,18 +219,29 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { MustBeImage = true });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, image.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(document.AssetId, image1.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "[1]: Not an image." });
         }
 
+        [Fact]
+        public async Task Should_add_error_if_values_contains_duplicate()
+        {
+            var sut = Field(new AssetsFieldProperties { MustBeImage = true });
+
+            await sut.ValidateAsync(CreateValue(image1.AssetId, image1.AssetId), errors, ctx);
+
+            errors.Should().BeEquivalentTo(
+                new[] { "Must not contain duplicate values." });
+        }
+
         [Fact]
         public async Task Should_add_error_if_image_width_is_too_small()
         {
             var sut = Field(new AssetsFieldProperties { MinWidth = 1000 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, image.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(document.AssetId, image1.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "[2]: Width \'800px\' less than minimum of \'1000px\'." });
@@ -221,7 +252,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { MaxWidth = 700 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, image.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(document.AssetId, image1.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "[2]: Width \'800px\' greater than maximum of \'700px\'." });
@@ -232,7 +263,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { MinHeight = 800 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, image.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(document.AssetId, image1.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "[2]: Height \'600px\' less than minimum of \'800px\'." });
@@ -243,7 +274,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { MaxHeight = 500 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, image.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(document.AssetId, image1.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "[2]: Height \'600px\' greater than maximum of \'500px\'." });
@@ -254,7 +285,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { AspectWidth = 1, AspectHeight = 1 });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, image.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(document.AssetId, image1.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[] { "[2]: Aspect ratio not '1:1'." });
@@ -265,7 +296,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         {
             var sut = Field(new AssetsFieldProperties { AllowedExtensions = ReadOnlyCollection.Create("mp4") });
 
-            await sut.ValidateAsync(CreateValue(document.AssetId, image.AssetId), errors, ctx);
+            await sut.ValidateAsync(CreateValue(document.AssetId, image1.AssetId), errors, ctx);
 
             errors.Should().BeEquivalentTo(
                 new[]
diff --git a/tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/ReferencesFieldTests.cs b/tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/ReferencesFieldTests.cs
index ace515b79..1e6c5ce66 100644
--- a/tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/ReferencesFieldTests.cs
+++ b/tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/ReferencesFieldTests.cs
@@ -61,6 +61,16 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
             Assert.Empty(errors);
         }
 
+        [Fact]
+        public async Task Should_not_add_error_if_duplicate_values_are_allowed()
+        {
+            var sut = Field(new ReferencesFieldProperties { MinItems = 2, MaxItems = 2, AllowDuplicates = true });
+
+            await sut.ValidateAsync(CreateValue(ref1, ref1), errors);
+
+            Assert.Empty(errors);
+        }
+
         [Fact]
         public async Task Should_add_error_if_references_are_required_and_null()
         {
@@ -127,6 +137,17 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
                 new[] { $"Contains invalid reference '{ref1}'." });
         }
 
+        [Fact]
+        public async Task Should_add_error_if_reference_contains_duplicate_values()
+        {
+            var sut = Field(new ReferencesFieldProperties { SchemaId = schemaId });
+
+            await sut.ValidateAsync(CreateValue(ref1, ref1), errors, ValidationTestExtensions.References(ref1));
+
+            errors.Should().BeEquivalentTo(
+                new[] { "Must not contain duplicate values." });
+        }
+
         private static IJsonValue CreateValue(params Guid[] ids)
         {
             return ids == null ? JsonValue.Null : JsonValue.Array(ids.Select(x => (object)x.ToString()).ToArray());