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 answers to your questions. Check out this post about deletion (see the remark).
I think it is clear that when someone answered your question, you are not the sole owner of the question anymore (indeed you never are). If you delete your question you also delete all the answers.
If you really want it to be deleted (see remark 2), you can post a suggestion in CRUDE chat room, and users with the deletion privilege might considering casting the delete vote (But your question is not of low quality and has already five answers, so I don't see why it should be deleted).
Remark: Let me just copy the relevant information in the link:
When can't I delete my own post?
Special note: You can't delete your own post using the Android app. You can, however, delete your post by using the app's option to open the question in a Web browser and deleting it from there, provided none of the below apply.
You can't delete your own answer if it has been accepted.
You can't delete your own question if it:
- has an answer with upvotes (even if that answer has a net zero or negative score)
- has an accepted answer
- has multiple answers (even if there are no upvotes)
- has an answer with an awarded bounty
You can't delete any of your questions or answers if you haven't registered your account (that is, associated a Google, Facebook, or Stack Exchange account with it).
Note: if you're a moderator, you can delete your own post even if it wouldn't ordinarily be allowed.
Remark 2 That does not work either as your question is a duplicate target of another question.