On an i386, in src/gnu/lib/libgcc, I get 2 source files per compile line for
eg. exception.cc (I think for all files mentioned in G_CXX_LIB2SRCS in
i386.mk). They differ by their path:

/usr/src/gnu/lib/libgcc/../../dist/toolchain/gcc/cp/exception.cc
/usr/src/gnu/lib/libgcc/../../dist/gcc/cp/exception.cc

extra "toolchain" in the first. Dropping the second (without toolchain)
source file from the compile line seems to work... Following the Makefile
magic is tricky..

Cheers,

Patrick