throw XamlParseException if there are duplicate property setters in style or control theme

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(),

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlDuplicateSettersChecker.cs

@@ -0,0 +1,44 @@
+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 fullName = objectNode.Type.GetClrType().FullName;
+        if (fullName is not ("Avalonia.Styling.Style" or "Avalonia.Styling.ControlTheme"))
+        {
+            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;
+    }
+}

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