[Users] Problems when branching multiple legs to asterisk

Hello list, Can anybody help me with the following problem: I have a network with a machine running OpenSER (1.1.0) and a machine running Asterisk (1.2.7). When a call is made to a certain number on the OpenSER machine I use a append_branch to fork the call to two phones, this works fine. The problem arises when I want to fork the call to two extensions on the asterisk machine. Only one of the extensions will actually start ringing. Using ethereal I found out that OpenSER sends two invites to the asterisk machine, asterisk returns a "100 Trying" on both invites, but after that Asterisk seems to forget about the second call. The first Invite is processed correctly. I suspect that the problem lies in the fact that asterisk receives two calls with the same Call-ID and just drops one of them. This behaviour is understandable since call-ids are supposed to be unique (as far as I know). Sending two invites with the same call-id on the otherhand also is the logical thing to do from OpenSERs point of view, but clearly this isn't "compatible". So my question is: Is there some way to let OpenSER succesfully branch a single call to two extensions on the same Asterisk? I hope somebody knows of a way how to fix this. Thanks in advance. Kind regards, Ardjan Zwartjes.