Your question is marked as duplicate, but this might not be the reason why you are getting a down-vote.
As pointed out in the comment, you have a net gain of reputation point, so in terms of reputation points, you are not being punished.
Yes you cannot delete your post, not because it is marked as duplicate. You cannot do that since there are upvoted ...
When you've got enough reputation that you can see deleted content, type the following command into the search bar: [gap] deleted:yes.
Searching for deleted content will only show content that you own, so you will immediately see your answers (and your questions if you asked any on the topic) and nothing more.