The bug was caused by the stripifier getting confused when faces were duplicated but with different vertex ordering.
I've added regression test to reproduce the situation of the nif & fixed it:
http://github.com/amorilia/pyffi/commit/73e68859c00669d200142f26832f40c043b54cfb
http://github.com/amorilia/pyffi/commit/48b5f2c4dbebe434b3b679e9d07847941e1192d2
As always,
Amorilia.
