[Serusers] SER with Multiple Contact

Hi all, I've configured a local SER on our network, and I met some difficulties with multiple Contacts. When UA1 changes IP address, and REGISTER in SER to announce his new IP, both last and New Contacts are placed in Location table, with different 'expires' values. If UA2 receives last UA1 IP address, and REGISTER with it, calling UA1 from UA3 when both Contact still available, makes UA2 ringing, which cause some confusion. To overcome about this, I'm searching for a way to force SER to delete an "old Contact" when a Registration is done with same URI and different IP; That way, only one Contact for one URI is available. I've tried to add modparam("registrar", "append_branches", 0) in ser.cfg, but in this case, Contact with lowest 'expires' parameter is called, and not Contact with highest one, still causing confusion fo IP calls. Could anyone tell me how to force a Contact to be deleted in SER location table, in case of registration with new Contact? Thanks for your support. Regards. Marc