Hi,
After some discussions on IRC we've decided to skip the sequence number checking for now and to not care about forwarding a packet to the higher layers more than once. Usually, higher layer protocols are in charge and should be capable of detecting and coping not only with lost, but also duplicate packets. I'm not aware of any protocol which might have any difficulties with this (if you know one, please let me know). If you have special, "custom" protocol which is not able to cope with it, then you're adviced to not activate the promisc-mode on that node's interface.
I also did some further testing on how usable such a feature on embedded devices is. Smaller, less powerful devices like the Fonera 2.0 can have some performance decreases in a very busy, noisy mesh neighborhood. All other scenarios and more performant devices should be fine with the promisc mode, however feel free to do your own measurments to find out if this promisc-mode feature is suitable and benefitial for you (and share your experiences :) ).
Cheers, Linus