Should I delete Questions?
No. You are not required to delete questions, but you have that choice (see Timelord64's answer for instructions on deleting). Systems are in place to delete questions that meet specific criteria (age, low/negative score, no answers etc).
Note, there is value in not deleting duplicates if the duplicates are worded differently. For example, How do I win? and How do I avoid defeat? questions could be duplicates of each other. I can google/search either form to get to the same set of answers. If you delete How do I win?, I may not find the answer to my question.