This would have been pretty unheard of; the system won't let you vote twice on a post with the same account. Was the upvote arrow already blue when you visited the page? Perhaps you realized, say, 6 months ago that your upvote wasn't warranted and did you retract it then already. The post's score might still be 1 because another user upvoted it.
In any case,...
Is double voting allowed?
The real answer is no! And I truly doubt that the event you described actually happened.
You can always edit the response you updated and then you will be able to remove your vote, if you truly feel the response does not merit being upvoted by yourself.