fix: add a workaround for net461's XmlAttributeCollection.InsertAfter

src/Uno.XamlMerge.Task/BatchMergeXaml.cs

@@ -254,8 +254,22 @@ private static bool TryMergeHashUsings(string existingNamespaceString, string ne
                         var ownerElement = propertyAttributeToUpdate.Key.OwnerElement;
                         var newAttribute = ownerElement.OwnerDocument.CreateAttribute(propertyAttributeToUpdate.Key.Prefix, propertyAttributeToUpdate.Key.LocalName, merged);
                         newAttribute.Value = propertyAttributeToUpdate.Key.Value;
-                        ownerElement.Attributes.InsertAfter(newNode: newAttribute, refNode: propertyAttributeToUpdate.Key);
-                        ownerElement.RemoveAttributeNode(propertyAttributeToUpdate.Key);
+
+                        var refNode = propertyAttributeToUpdate.Key;
+
+                        if (newAttribute.Name == refNode.Name || newAttribute.NamespaceURI == refNode.NamespaceURI)
+                        {
+                            // This is a workaround for a bug in net461's InsertAfter that crashes if refNode has the same name as newAttribute
+                            // If refNode and newAttribute are duplicates, it doesn't matter whether we insert before or after,
+                            // since refNode will be removed before the insertion anyway.
+                            ownerElement.Attributes.InsertBefore(newNode: newAttribute, refNode);
+                        }
+                        else
+                        {
+                            ownerElement.Attributes.InsertAfter(newNode: newAttribute, refNode);
+                        }
+
+                        ownerElement.RemoveAttributeNode(refNode);
                     }
                 }
 

src/Uno.XamlMerge.Tests/Given_BatchMergeXaml.cs

@@ -42,7 +42,9 @@ public void When_Different_Properties_Different_Namespaces()
     {
         var task = CreateMerger();
 
-        task.Execute(); // Shouldn't throw an exception
+        task.Execute();
+
+        ValidateOutput(task);
     }
 
     [TestMethod]

src/Uno.XamlMerge.Tests/Scenarios/When_Different_Properties_Different_Namespaces/expected.xml

@@ -0,0 +1,10 @@
+<ResourceDictionary xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="skia" xmlns:skia="http://uno.ui/skia" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
+  <!--origin: When_Different_Properties_Different_Namespaces\Input_Dictionary.xml-->
+  <Style TargetType="ScrollViewer">
+    <Setter Property="Template">
+      <Setter.Value>
+        <ScrollBar Height="16" skia:Width="16" />
+      </Setter.Value>
+    </Setter>
+  </Style>
+</ResourceDictionary>

src/Uno.XamlMerge.Tests/Uno.XamlMerge.Tests.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>net6.0;net461</TargetFrameworks>
-    <LangVersion>11</LangVersion>
+    <LangVersion>10</LangVersion>
     <Nullable>enable</Nullable>
 
     <IsPackable>false</IsPackable>