[Freeswitch-users] re-invite problem

I am running a b2bua with freeswitch. It is fine until a Mitel UAS starts sending INVITE without sdp and ACK with sdp. Freeswitch seems to treat it as another dialog and sends it to dialplan handling. Within the dialplan, how can I recognize request as a re-invite and possibly ignore it? Does anyone know how to resolve this problem?