I opened a bug report (10k only) question in MSE yesterday which is clearly a duplicate of a deleted question (10k only). As I do not have the reputation to see deleted questions, I went on and filed the report. It gets closed as too localized.
The original question was asked some (four) hours before (according to this comment (waffle page now)) and had been deleted. Had it not been deleted, I would have seen it and not have created the duplicate question.
I'm not sure this is something that happens regularly, but it could be prevented easily by letting 'solved' questions be visible for 48hours before elimination.
Edit: Robert Harvey states that SE was working on this specific issue right after it happened. That's cool. I'm saying that the easiest way to let everybody be aware of the work in progress is to keep the very first question around with the answer (we are working on this). That way duplicates are prevented.