Multiple registration using same user ID

Hi

Plz see the following steps. And cleare my doubt

  1. “User A” registered with asterrisk server first which is present in machine 1.

  2. “User A” (Same user)registered with asterrisk server (Second) which is present in machine 2.

  3. Now “User B” Calls “user A”.

  4. some times the “UserA” in machine 1 gets the call , Some times userB in machine 2 is gets the call.

How it happens. Is there any settings in sip.conf file to control it. ? reply plz

Regards,
Amsa