Duplicate SSH Keys

As I'm going through the SSH key validation code in order to add the auto-conversion for ssh2 keys and detection of private keys, etc I'm wondering.. Do we want to enforce a rule preventing multiple people from having the same pubic key? I'm trying to think of a situation which would be valid for different identities to share a key, and I'm coming up blank.. Right now, there are 7 non-deleted duplicate keys in the system, and none of them are shared.. (ie, they've uploaded it twice, and these are accounts which have had issues setting their keys) Thoughts?