Fix #1599: Ignore duplicate keys in resource dictionary.

ILSpy.BamlDecompiler/Rewrite/MarkupExtensionRewritePass.cs

@@ -77,6 +77,7 @@ namespace ILSpy.BamlDecompiler.Rewrite {
 			var attrName = elem.Name;
 			if (attrName != key)
 				attrName = property.ToXName(ctx, parent, property.IsAttachedTo(type));
+			if (!parent.Attributes(attrName).Any()) {
 				var attr = new XAttribute(attrName, extValue);
 				var list = new List<XAttribute>(parent.Attributes());
 				if (attrName == key)
@@ -85,6 +86,7 @@ namespace ILSpy.BamlDecompiler.Rewrite {
 					list.Add(attr);
 				parent.RemoveAttributes();
 				parent.ReplaceAttributes(list);
+			}
 			elem.Remove();
 
 			return true;