How can I duplicate and upload an already uploaded mario maker course?

Once you upload a level, it gets marked as uploaded (with a small up arrow icon in the level selection menu), and cannot be reuploaded unless you delete the uploaded level from course world.

Even if you save a copy of the level in a new slot after uploading it, the marker will stay and the copy also cannot be uploaded unless the original level is first deleted from course world.

However, if you make a copy of your level before uploading it, the copy will not be marked as uploaded. So you can in fact upload as many different versions of your level as you like, as long as you remember to always save a backup copy before you upload the level.

Alas, if you already uploaded your level and forgot to make a backup copy, there's nothing (AFAIK) you can do except delete the level from course world. At least, when you do that, all copies of the level that you've saved after uploading it will have the upload marker removed from them, so you can then reupload each of them as separate levels.

^{(This answer is based on testing I just did in MM2. My apologies to everyone who ended up playing my awful test levels.)}

In short, you can't .

All you can do is try to delete the previous upload course......