The whole UI for closing a question as an exact duplicate is buggy and annoying. It's hard for me to describe the pain and angst I feel every time I vote to close for exact duplicate. Every time I do it I'm going to record a movie and post it in this question. Here's the most recent time I've done it:
I still don't know how to use this UI correctly. Anything I do doesn't seem to work other than in a random fashion. It sucks, and I demand it be fixed immediately. Harumph.