You have two files that define 'WDT_Highlight". Most likely Quartz modified a file that the defintion was originally located. Then it was moved to another file that Quartz did not modify, which results in a duplicate.
You need to do a text search in your UI folder and find out which file(s) have the definition it it and decide which one to delete it from.
Oh, and Silver does not restrict UI selection in any way. Some stuff won't show, but that is not by UI and just because of silver (for example, you will only see 4 bags in inventory)