Cloudinary provides you with two powerful criteria for determining duplications - one for identity comparison and the other for similarity:
MD5 signature: The
"etag"attribute included in the upload response can be used for recognizing completely identical images.
pHash: If requested (
"phash" => true) this will be included in the upload response, allowing to commit a similarity check and distinguish how close the images are.
More information on pHash: http://www.phash.org/demo/
Surrogate Key: If the images are served through Akamai CDN then Surrogate Key can be used to check for duplication.
Checking image dimensions is always a good idea too. Using different hashes instead of just one reduces the chance of hash collisions.