Storing unencrypted file hashes is the easiest way I can see to do it; then, for deduplication, the file is decrypted using one user's key, and a copy is made a

Storing unencrypted file hashes is the easiest way I can see to do it; then, for deduplication, the file is decrypted using one user's key, and a copy is made and encrypted under the second user's key.