Well, if they are really duplicates and they have no answer, they are probably not of that much value to the community. Hence you could delete them. Note that duplicates has a significant value in finding the duplicate question. Your question might contain keywords the original doesn't have, so keep them if you wish.
I should warn you though that asking multiple duplicate questions may lead to a question ban (deleting posts do count toward a question ban too). The problem with those questions is: there usually isn't much to improve so they get reopened which may get you out of a ban.
So instead of worrying whether you should delete them or not, worry about asking questions that are a duplicate. That is much worse than actually deleting them.