When you first voted for the question as it was a nice question and had nice answers, you used your vote correctly: the question was clear and nicely written. But now, after seeing that it's a duplicate and wanting to retract the vote just because it is a duplicate isn't doing the question justice.
Reading Jeff's blog about the types of duplicates and their handling gives a good idea about the types of duplicate. The article also hints that not all duplicates are bad.
Doing some homework before casting a vote, in my opinion, is always a good thing to do. If not for duplicates atleast make sure that it isn't:
- Primarily opinion based.
- Too broad.
Then, do a quick check for duplicates, perhaps in the sidebar, using the site-search and using google. If you find it quickly and the question has good answers, vote to close the question as an duplicate. If it has bad answers, you can wait for new answers and vote to close the original question as an duplicate.
However if the duplicate was too hard to find, and for some reason you couldn't find it even after thoroughly searching and someone else later points out in the comments that it is a duplicate, upvote the question. You now have a better coverage and a greater chance of new users finding the question.