A 'good' hash function (exhibiting uniformity) that converts arbitrary-sized documents to finite-sized hashes should not have any hashes that occur precisely once.
This can occur with ill-conceived hash functions, for example if it bundles the length of the document with the hash, so there would be only 256 distinct hashes with length=1.
This can occur with ill-conceived hash functions, for example if it bundles the length of the document with the hash, so there would be only 256 distinct hashes with length=1.