If I ask a question and a reviewer comments that it's a duplicate, shall I delete the question before it is answered and deletion becomes unavailable?
The help center explains this:
Some duplicate questions may eventually be deleted, but often they are left as a signpost pointing people towards the canonical answer to that question.
In other words, you should let the question remain so that it will be easier for others to find the way to the previous answer. The fact that you didn't find it when you did your own search on the site before posting would seem to indicate that such a sign post is indeed useful.