On Thu, Dec 4, 2014 at 1:49 PM, Daniel Kinzler <daniel@brightbyte.de> wrote:

This is happening automatically: the SHA1 hash of every file is computed on
upload, and placed in the img_sha1 field on the database. I believe this is used
to warn users who try to upload an exact duplicate, but I'm not sure this is
true.

Indeed; so chances are exact duplicates have been uploaded intentionally. (It's not certain because they might have been uploaded before this warning existed, or uploaded by a bot configured to ignore warnings.)

There is also an automated report: https://commons.wikimedia.org/wiki/Special:ListDuplicatedFiles