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    I think another side effect would be the requirement of long cat picture URLs :P Commented Dec 13, 2015 at 17:14
  • People can not upload exact duplicate images. Open a .jpg, "save as", and hash both files. It's possible that your software spots there's no change and writes the original data, but tweak one pixel in any image format and the hash will be different. So you don't even reliably prevent people uploading the same image accidentally given that the "same" image may not be the same file (e.g. automagic resizing on mobile). Image hashing would fix that, but isn't a secure hash. It may not matter of course. Commented Dec 14, 2015 at 9:15
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    @Chris with real-world image hosting services (like Imgur, for instance), a common use case is taking an image file obtained elsewhere and uploading it as-is, with no editing at all, not changing a single pixel, not even opening it in an image editor and hitting "Save As". It's probably quite common that people upload bit-for-bit identical images. Commented Dec 14, 2015 at 19:33
  • @DavidConrad, I'm sure it is common. Re-uploading a non-bit-identical image probably also is: on http my mobile provider sometimes recompresses images (making e.g. maps useless in the process). Re-upload that and it won't hash the same, with no user action to alter it. Commented Dec 14, 2015 at 21:58
  • The point is that the statement People can not upload exact image duplicates is wrong. Commented Dec 17, 2015 at 15:54