Yes it already stops you from uploading hash duplicates but only after you've already entered in all the info and tags and such just to find out it's already on the site but no one bothered to tag it properly.
It would be convenient if it could after the image was selected either automatically check or give the option to manually check if duplicate or similar images already exist.
Even just having a link to the similar images search page from somewhere on the upload page would be convenient.