If a question is marked as a duplicate or is closed, should I delete it?
I really don't know if I can clarify that any more.
Nope, leave it in place. Someone could search with phrasing more similar to what you used than the original.
You should not delete questions closed as a duplicate. Those serve as signposts to the primary question and can help those thinking of the question in a different way discover it.
You could delete questions closed for other reasons - but why? If the system wanted them deleted for being closed, they’d get deleted once on hold turns to closed. Note that the system may even stop you from deleting them if they have upvoted answers.