[Serusers] double INVITE in a single call

I have several calls that have two branches, one in one way and another in the other way, I mean, for the same call ID, user 1 calls user 2, and also, user 2 calls user 1.

I wonder if this is normal, or if I have a something wrong. This only occur sometimes, so it isn´t a routing problem.

I have this problem in different scenarios, so it has to do with ser configuration, not with the user routing scheme. Also, I have this problem with calls between sip users or calls between sip user and PSTN.

For the same call, I´ve got two logs:

User 1 -> User 2 INVITE fromtag: id1 totag: id2 CALLID1 User 2 -> User 1 INVITE fromtag: id2 totag: id1 CALLID1

Where id1 and id2 are the same identifier.

Can anybody help me?

thanks a lot!!