diff --git a/src/Squidex/app/framework/angular/forms/error-formatting.spec.ts b/src/Squidex/app/framework/angular/forms/error-formatting.spec.ts
index f6ac790c3..88b4206ad 100644
--- a/src/Squidex/app/framework/angular/forms/error-formatting.spec.ts
+++ b/src/Squidex/app/framework/angular/forms/error-formatting.spec.ts
@@ -132,6 +132,12 @@ describe('formatErrors', () => {
         expect(error).toEqual('MY_FIELD contains an invalid value: 4.');
     });
 
+    it('should format uniqueStrings', () => {
+        const error = validate(['1', '2', '2', '3'], ValidatorsEx.uniqueStrings());
+
+        expect(error).toEqual('MY_FIELD must not contain duplicate values.');
+    });
+
     it('should format match', () => {
         const formControl1 = new FormControl(1);
         const formControl2 = new FormControl(2);
diff --git a/src/Squidex/app/framework/angular/forms/error-formatting.ts b/src/Squidex/app/framework/angular/forms/error-formatting.ts
index a54d23941..1b479eb8e 100644
--- a/src/Squidex/app/framework/angular/forms/error-formatting.ts
+++ b/src/Squidex/app/framework/angular/forms/error-formatting.ts
@@ -26,6 +26,7 @@ const DEFAULT_ERRORS: { [key: string]: string } = {
     patternmessage: '{message}',
     required: '{field} is required.',
     requiredTrue: '{field} is required.',
+    uniquestrings: '{field} must not contain duplicate values.',
     validdatetime: '{field} is not a valid date time.',
     validvalues: '{field} is not a valid value.',
     validarrayvalues: '{field} contains an invalid value: {invalidvalue}.'
diff --git a/src/Squidex/app/framework/angular/forms/validators.spec.ts b/src/Squidex/app/framework/angular/forms/validators.spec.ts
index 1e0911720..9cd351171 100644
--- a/src/Squidex/app/framework/angular/forms/validators.spec.ts
+++ b/src/Squidex/app/framework/angular/forms/validators.spec.ts
@@ -371,4 +371,38 @@ describe('ValidatorsEx.pattern', () => {
 
         expect(error).toEqual(expected);
     });
+});
+
+describe('ValidatorsEx.uniqueStrings', () => {
+    it('should return null when value is null', () => {
+        const input = new FormControl(null);
+
+        const error = ValidatorsEx.uniqueStrings()(input);
+
+        expect(error).toBeNull();
+    });
+
+    it('should return null when value is not a string array', () => {
+        const input = new FormControl([1, 2, 3]);
+
+        const error = ValidatorsEx.uniqueStrings()(input);
+
+        expect(error).toBeNull();
+    });
+
+    it('should return null when values are unique', () => {
+        const input = new FormControl(['1', '2', '3']);
+
+        const error = ValidatorsEx.uniqueStrings()(input);
+
+        expect(error).toBeNull();
+    });
+
+    it('should return error when values are not unique', () => {
+        const input = new FormControl(['1', '2', '2', '3']);
+
+        const error = ValidatorsEx.uniqueStrings()(input);
+
+        expect(error).toEqual({ uniquestrings: false });
+    });
 });
\ No newline at end of file
diff --git a/src/Squidex/app/framework/angular/forms/validators.ts b/src/Squidex/app/framework/angular/forms/validators.ts
index a0c61f551..3a08ed7c4 100644
--- a/src/Squidex/app/framework/angular/forms/validators.ts
+++ b/src/Squidex/app/framework/angular/forms/validators.ts
@@ -7,7 +7,7 @@
 
 import { AbstractControl, ValidatorFn, Validators } from '@angular/forms';
 
-import { DateTime } from '@app/framework/internal';
+import { DateTime, Types } from '@app/framework/internal';
 
 function isEmptyInputValue(value: any): boolean {
     return value == null || value.length === 0;
@@ -175,4 +175,25 @@ export module ValidatorsEx {
             return null;
         };
     }
+
+    export function uniqueStrings(): ValidatorFn {
+        return (control: AbstractControl) => {
+            if (isEmptyInputValue(control.value) || !Types.isArrayOfString(control.value)) {
+                return null;
+            }
+
+            const a: string[] = control.value;
+            const unique: { [key: string]: boolean } = {};
+
+            for (let value of a) {
+                if (unique[value]) {
+                    return { uniquestrings: false };
+                } else {
+                    unique[value] = true;
+                }
+            }
+
+            return null;
+        };
+    }
 }
\ No newline at end of file