I finished writing a question, and then realized it is a duplicate. Should I post?
I swear this happens all the time to me.
Reason not to post: it is a duplicate, and will waste peoples' time with the question (having been bumped to the top). Perhaps this can be mitigated by the OP immediately flagging it as a duplicate.
Reason to post: for the same reasons we don't delete well-posed duplicates; e.g. to improve the chances someone searching for this question will be able to find it.
(It would have been pretty ironic if I had found out this question was a duplicate before posting.)