Just what the title says: I've had many questions marked as duplicates when I first started using this site, and I was wondering if it's okay for me to delete them.
It depends. Ultimately it's your decision whether you want to delete your unanswered duplicate questions - within the constraints of the software: you can't delete more than five of your own posts per day; and deleted posts can be undeleted by other users, though for unanswered duplicate questions that's rather unlikely to happen.
But some duplicate questions serve a useful purpose. If duplicates ask the same question in a different way, that gives a larger hit-area for future searchers. They will then find the answer to their question via different searches, and so having several differently phrased versions of the same question on the site reduces the number of future duplicates asked.
So I would recommend comparing your question to the duplicate target, and other duplicates of that, if there are some. And look at the view count. If your question differs from the other versions, or it has a high view count, then it is probably a useful question to keep on the site. If your question is for all practical purposes identical to another version, and it has low view count, there's probably little value in keeping it around.
Look at your questions and make your judgement.