It's really not a problem. Your application would just need to keep track of who has which keys, and with this approach it already does. When a user attempts to check-in a key, you validate against the user_id, so 'select id from key where user_id=? and opens=? and checked_out=1', and then let them check-in the first row that returns from that query. If no rows appear, they cannot check a key in, because it either means they already checked it in, or never had one checked out to begin with.