accel-ppp.org

If a client normally connects via PPPoE and then also connects (via a different route) with an L2TP client both connections will get the same IP address (assuming assigned via Radius)

Is there anyway to set the routing metric on a per protocol level? e.g. if I want to packets to always return on the L2TP connection if both are up?

Interesting, maybe it is possible to reach with pppd_compat and run in ip-up script something like
ip route replace {IP} dev {DEVICE} metric {METRIC}
For different proto different metrics. It is just an idea