SnapBill always maintains two references to every object on your account. The objects id field will always be unique for an account.
Due to our systems design we do not guarentee that the id field will be unique across accounts. For example if you signup to SnapBill twice, and add clients to each account - occasionally a client on the one account will land up with the same id as a client on the other.
If you are storing data on multiple accounts we recommend either grouping your data by account, or simply using the xid parameter which is guarenteed to be unique across every single SnapBill account.