First, those two options overlap. The post is a duplicate and I am the uploader. Both are true. And it's a choice of one. Basic UI fail.

Then it makes sense to pick the most specific option. Anyone can flag it a duplicate. Only the uploader, and only for the first 48 hours, can use the uploader option, so it's more specific.

There's no way to change, retract, or preview the flag. The uploader flags the post, looks at the result and goes like nope, that's not right, that's not what I expected. But there's way to change anything at this point.

Finally, certain site rules have consequences. Uploading a dupe is a punishable offence on e6. It adds to the negative counter. It's up there with commercial content and irrelevant posts. Also thanks BVATS most people are aware of this rule. Fixing your own mistake quickly, well, you shouldn't get punished for a mis-click or flaky connection or whatever right? If you fixed it on your own? So it's one more point towards the custom reason.