There are two cases in the last hour of questions which have appeared twice, with both copies appearing pretty much simultaneously:

313547 = 313548

313550 = 313551

Coincidence or bug?

EDIT: I posted this question about 20 minutes ago and got an error message saying that it couldn't be posted (I don't remember the exact words). I didn't try again, but just noticed that my question has subsequently appeared. I'm guessing that the posters of the questions that I noticed on the main site did try again ...