I asked a question a couple of years ago on MathStackexchange which was quite popular. I would like to ask it on MathOverflow as well, since I'd be interested in the answers there. But I'm never sure how the cross-posting rules work.
I understand that lots of overflow users still see stackexchange anyway, particularly if it was a quite popular question, but I still feel there'd be a decent portion who can provide new answers.
The question is quite subjective, since essentially I'm just asking for an example that personally satisfies me, so the question of "whether it has an answer already" is a bit difficult. In any case, I feel I don't have an answer.