Strange pppoe problem - duplicate packages

Trying to debug a problem at customer site(don't have any tools really) what seems to
be some sort of duplication of pppoe pkgs.

I got 2 pppoe links going out on the same eth0 I/F to 2 different MAC addresses but with the
same session ID.

pinging one neighbour results in 2 replies, one from each pppoe neighbour!
It looks like linux is ignoring the different MAC addresses somehow and just uses
the session ID and since it is the same session ID, I get 2 pkgs.

Does this sound familiar to anyone? Any idea where to start looking? I suspect the
TX patch somehow duplicating the pkgs.

 Jocke

PS.
    My kernel is somewhat old, 2.6.35, but I have checked current sources and it
    doesn't look like there has been any significat changes in this area.