After noticing that our site hasn't had a challenge on the Coin Problem, I recently posted one. However, it was pointed out that the challenge was posted on Stack Overflow back in the day when codegolf.SE didn't yet exist and such challenges were accepted on SO. The old challenge has a fair amount of answers as well. Should I close my challenge as a duplicate of another question on a Stack Exchange site, or let it be?
Let it be. Codegolf questions on SO get deleted apparently at random, so there's no telling how long that one will be there. However, it might be a good idea to link to it from your question and to state that if people want to copy answers then they should credit the original poster, make their answers community wiki, and be prepared to delete them if the original poster wants to repost them here.