diff --git a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlCompiler.cs b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlCompiler.cs
index aaaee39b0d..197815f9a0 100644
--- a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlCompiler.cs
+++ b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlCompiler.cs
@@ -49,6 +49,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
             InsertBefore<ContentConvertTransformer>(
                 new AvaloniaXamlIlControlThemeTransformer(),
                 new AvaloniaXamlIlSelectorTransformer(),
+                new AvaloniaXamlIlDuplicateSettersChecker(),
                 new AvaloniaXamlIlControlTemplateTargetTypeMetadataTransformer(),
                 new AvaloniaXamlIlBindingPathParser(),
                 new AvaloniaXamlIlPropertyPathTransformer(),
diff --git a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlControlThemeTransformer.cs b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlControlThemeTransformer.cs
index 1338dc7248..51fe58d1c9 100644
--- a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlControlThemeTransformer.cs
+++ b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlControlThemeTransformer.cs
@@ -11,7 +11,8 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
     {
         public IXamlAstNode Transform(AstTransformationContext context, IXamlAstNode node)
         {
-            if (!(node is XamlAstObjectNode on && on.Type.GetClrType().FullName == "Avalonia.Styling.ControlTheme"))
+            if (node is not XamlAstObjectNode on ||
+                !context.GetAvaloniaTypes().ControlTheme.IsAssignableFrom(on.Type.GetClrType()))
                 return node;
 
             // Check if we've already transformed this node.
diff --git a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlDuplicateSettersChecker.cs b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlDuplicateSettersChecker.cs
new file mode 100644
index 0000000000..4ab9594cd8
--- /dev/null
+++ b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlDuplicateSettersChecker.cs
@@ -0,0 +1,45 @@
+using System.Collections.Generic;
+using System.Linq;
+using XamlX;
+using XamlX.Ast;
+using XamlX.Transform;
+
+namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers;
+
+class AvaloniaXamlIlDuplicateSettersChecker : IXamlAstTransformer
+{
+    public IXamlAstNode Transform(AstTransformationContext context, IXamlAstNode node)
+    {
+        if (node is not XamlAstObjectNode objectNode)
+        {
+            return node;
+        }
+
+        var nodeType = objectNode.Type.GetClrType();
+        if (!context.GetAvaloniaTypes().Style.IsAssignableFrom(nodeType) &&
+            !context.GetAvaloniaTypes().ControlTheme.IsAssignableFrom(nodeType))
+        {
+            return node;
+        }
+
+        var properties = objectNode.Children
+            .OfType<XamlAstObjectNode>()
+            .Where(n => n.Type.GetClrType().Name == "Setter")
+            .SelectMany(setter =>
+                setter.Children.OfType<XamlAstXamlPropertyValueNode>()
+                    .Where(c => c.Property.GetClrProperty().Name == "Property"))
+            .Select(p => p.Values[0])
+            .OfType<XamlAstTextNode>()
+            .Select(x => x.Text);
+        var index = new HashSet<string>();
+        foreach (var property in properties)
+        {
+            if (!index.Add(property))
+            {
+                throw new XamlParseException($"Duplicate setter encountered for property '{property}'", node);
+            }
+        }
+
+        return node;
+    }
+}
diff --git a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlSelectorTransformer.cs b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlSelectorTransformer.cs
index 078e23bc02..edddc5424a 100644
--- a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlSelectorTransformer.cs
+++ b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlSelectorTransformer.cs
@@ -19,7 +19,8 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
     {
         public IXamlAstNode Transform(AstTransformationContext context, IXamlAstNode node)
         {
-            if (!(node is XamlAstObjectNode on && on.Type.GetClrType().FullName == "Avalonia.Styling.Style"))
+            if (node is not XamlAstObjectNode on ||
+                !context.GetAvaloniaTypes().Style.IsAssignableFrom(on.Type.GetClrType()))
                 return node;
 
             var pn = on.Children.OfType<XamlAstXamlPropertyValueNode>()
diff --git a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlWellKnownTypes.cs b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlWellKnownTypes.cs
index 16f6a32ae1..92710e42ce 100644
--- a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlWellKnownTypes.cs
+++ b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlWellKnownTypes.cs
@@ -112,6 +112,8 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
         public IXamlMethod ResourceDictionaryDeferredAdd { get; }
         public IXamlType UriKind { get; }
         public IXamlConstructor UriConstructor { get; }
+        public IXamlType Style { get; }
+        public IXamlType ControlTheme { get; }
 
         public AvaloniaXamlIlWellKnownTypes(TransformerConfiguration cfg)
         {
@@ -250,6 +252,8 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                     XamlIlTypes.Object));
             UriKind = cfg.TypeSystem.GetType("System.UriKind");
             UriConstructor = Uri.GetConstructor(new List<IXamlType>() { cfg.WellKnownTypes.String, UriKind });
+            Style = cfg.TypeSystem.GetType("Avalonia.Styling.Style");
+            ControlTheme = cfg.TypeSystem.GetType("Avalonia.Styling.ControlTheme");
         }
     }
 
diff --git a/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/XamlIlTests.cs b/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/XamlIlTests.cs
index f42f787117..be2cae8ec4 100644
--- a/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/XamlIlTests.cs
+++ b/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/XamlIlTests.cs
@@ -334,6 +334,47 @@ namespace Avalonia.Markup.Xaml.UnitTests
             var parsed = (Button)AvaloniaRuntimeXamlLoader.Load(document);
             Assert.Equal(Colors.Blue, ((ISolidColorBrush)parsed.Background!).Color);
         }
+
+        [Fact]
+        public void Style_Parser_Throws_For_Duplicate_Setter()
+        {
+            var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.Xaml;assembly=Avalonia.Markup.Xaml.UnitTests'>
+    <Window.Styles>
+        <Style Selector='TextBlock'>
+            <Setter Property='Width' Value='100'/>
+            <Setter Property='Height' Value='20'/>
+            <Setter Property='Height' Value='30'/>
+        </Style>
+    </Window.Styles>
+    <TextBlock/>
+</Window>";
+            AssertThrows(() => AvaloniaRuntimeXamlLoader.Load(xaml, typeof(XamlIlTests).Assembly, designMode: true),
+                e => e.Message.StartsWith("Duplicate setter encountered for property 'Height'"));
+        }
+
+        [Fact]
+        public void Control_Theme_Parser_Throws_For_Duplicate_Setter()
+        {
+            var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:u='using:Avalonia.Markup.Xaml.UnitTests.Xaml'>
+    <Window.Resources>
+        <ControlTheme x:Key='MyTheme' TargetType='u:TestTemplatedControl'>
+            <Setter Property='Width' Value='100'/>
+            <Setter Property='Height' Value='20'/>
+            <Setter Property='Height' Value='30'/>
+        </ControlTheme>
+    </Window.Resources>
+
+    <u:TestTemplatedControl Theme='{StaticResource MyTheme}'/>
+</Window>";
+            AssertThrows(() => AvaloniaRuntimeXamlLoader.Load(xaml, typeof(XamlIlTests).Assembly, designMode: true),
+                e => e.Message.StartsWith("Duplicate setter encountered for property 'Height'"));
+        }
     }
 
     public class XamlIlBugTestsEventHandlerCodeBehind : Window