Not sure if your clients your the client interface or email them in. We use email and have seen sometimes where it doesn't recognize the client as existing and creates multiple clients with the same address. When we delete all instances of the client, their emails come through as they should. Also, verify that the domain settings aren't limiting non-standard emails from being processed- there are "white lists" and "black lists" to ensure only the right emails come in.
I hope these help...