[Serusers] forking error - branch response sent to uac

hi, I encountered following error. I have two user agents register under the same uid. When there is an invite request both ua's ring. When one of them UA1 answers the call, by sending 200 response some strange error occurs. UA1's 200 response is immediately sent to UAC who initiated a call. after this ser sends cancel message to other UA2. The other UA2 responses with 200 to the cancel message and sends response to the initial invite: 487 Request Terminated. After this ser should response with ACK to the second UA2 but it isn't instead it forwards 487 response to UAC who sent the initial invite. Please tell why ser behaves this way? What could be the reason for such behavior? Regards Tomasz