Mate problem in creating Gop with duplicated AVPs

edit

I'm playing with MATE and as a start point I'm implementing the HTTP grouping as per tutorial. With the code form the example no Gop is created (only Pdu) but if instead I match on a single addr port it works

Gop http_req On http_pdu Match (addr, port) {
    Start (http_rq);
    Stop (http_rs);
};

I think the example is correct but I don't understand why it does not work.

EDIT: this happens when src and dst IP are the same