When uploading an image already present on danbooru, the user is presented with a page explaining that the file already exists. "This upload is a duplicate: post #xxxxxx"
The problem is that the tags entered for the new image have been merged with the existing post. This is usually not a problem for correctly sourced duplicate images, e.g. finding an image on pixiv and not realizing it's already on danbooru. However, it is a problem if the source link is wrong - this morning I uploaded an image set, using the browser's back button to easily duplicate tags. On two occasions I forgot to advance the page number in the source link, resulting in tags for the new image being applied to the previous image.
I feel that, rather than merging tags automatically, the duplicate upload page should ask the user if they want to merge the new tags with the existing file. Ideally, it would even list out which tags would be added. If nothing else, the current page should at least warn the user that the tags have been merged, as there is currently no mention of this fact.