Is there a way to undo a vote to close a question because you thought it was a dupe and then realized it wasn't?
This has been requested on the main meta site, and declined there.
Jeff's explanation was:
declining -- you can always cast a reopen vote if the post gets closed.
Also note that all close votes automatically expire after two days.
(and for that matter reopen votes, or any other vote that attempts to reach a threshold -- otherwise, over an absurdly long period of time, say 10 years, everything would reach the threshold eventually through a tiny trickle of accumulated votes)
See here for when and how close votes expire: