I sympathize. I have purchased two outfits by mistake, that I have no particular use for, exactly the same way. In my case, I was in a hurry, the game was lagging, i was clicking in irritation and I ended up spending a bunch of diamonds I wish I hadn't.
Taught me to avoid extra clicks while irritated. Good life lesson I guess, but I could have done without.
Your only possible recourse in this situation is to post a ticket asking for your diamonds back, but generally that has not been successful in the past. I wish I had a better answer for you.