Gergo Tisza, 04/12/2014 23:27:
On Thu, Dec 4, 2014 at 1:49 PM, Daniel Kinzler 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.)
This warning is shown at Special:Upload. I don't remember if all users have the permission to click the "upload anyway"/"ignore warnings" button. On UploadWizard, the upload just fails in the "upload" step and I can only remove the file.
It's possible users didn't understand that duplicate means byte by byte duplicate, but I checked a dozen examples and most seem result of an upload failure. For instance: two pages of a book are given the same image; an image uploaded twice within 3 minutes (and used only with the second title).
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