I want to delete a question I asked but I don't know exact consequence. I'm going to try to delete a question I asked that was a duplicate (but it's not closed, it's just marked as duplicate). I don't see much reason to keep a duplicate, do you?
There is no ban for deleting a single post. In fact deleting your own question or answers when you realize they may not be helpful is encouraged. This will help to keep the site clear.
There is also no ban or other sanction in case you decide to delete your duplicate question but in general it may be better here to leave them as a signpost for other users which may help to find the answer given in the master question to which it will be linked to after closing.
Only in rare cases when we see users deleting many of their posts with no obvious reason this may be considered as vandalism, and it may eventually lead to consequences.
A duplicate contributes to the master question's links on the site (and FAQ-ability), and it also acts as an automatic redirect page to the master question when a user comes in from a search engine.
So in other words, it's a sign post that points people in the right direction. I would just leave it, shrug.