I've had it happen more times than I can count where I uploaded an image from one source, only to discover very shortly after that there is a better image available at another source and/or post. I don't really like doing double uploads like that so close to each other in time, so that usually leaves me with two options.
Options
1. Upload the better version and flag the inferior version
I'll do that one if needed, and usually only if the image dimensions are bigger than the other, but it's a bit awkward having your mistake being shown for all of Danbooru to see for 3 whole days until it finally gets deleted.
2. Don't upload the better version
If the images are the same dimensions, I'll usually just bite the bullet and swallow that mistake, but it always grinds my gears.
Considerations
The above is fine for contributors, since they get to upload pretty much what they want, but there would need to be restrictions for non-contributors, so that they don't pull some bait-and-switch. Maybe the approver of an image could have that function in the first hour after approval?
Anyways, thoughts?