I had a problem like this where I would sometimhes get
"serial line is looped back".  What I did was change my PPP
script so that, on the remote end, I would start pppd, then,
about 2 seconds later, emit "Starting PPP", and chat would wait for that.

My problem is that it was "succeeding" and starting to communicate
before the remote pppd was ready, so I was sending to a shell which,
of course, echoed characters.

YMMV.

-s