Throw for duplicate setters in styles.

And remove the duplicate setter that we have.

src/Avalonia.Base/Styling/StyleInstance.cs

@@ -45,7 +45,12 @@ namespace Avalonia.Styling
         public void Add(ISetterInstance instance)
         {
             if (instance is IValueEntry valueEntry)
-                base.Add(valueEntry);
+            {
+                if (Contains(valueEntry.Property))
+                    throw new InvalidOperationException(
+                        $"Duplicate setter encountered for property '{valueEntry.Property}' in '{Source}'.");
+                Add(valueEntry);
+            }
             else
                 (_setters ??= new()).Add(instance);
         }

src/Avalonia.Themes.Fluent/Controls/ButtonSpinner.xaml

@@ -53,7 +53,6 @@
 
   <!--  ButtonSpinner  -->
   <Style Selector="ButtonSpinner">
-    <Setter Property="Background" Value="Transparent" />
     <Setter Property="Foreground" Value="{DynamicResource TextControlForeground}" />
     <Setter Property="Padding" Value="10, 0" />
     <Setter Property="Background" Value="{DynamicResource TextControlBackground}" />