Question on repeating -march flags

[Yuri D'Elia]

I couldn't find anything on the matter, but what happens if -march is
repeated more than once?  I would assume the usual behavior that the
last flag "wins".

On a haswell host, the following:

gcc -march=native -march=opteron

or

gcc -march=opteron -march=native

both emit code which is illegal for the opteron ISA, as if -march=native
had special treatment. Specifying -march=opteron alone works as intended.

Since I generally override the default flags in makefiles by appending
exceptions where needed, I'd sort of expected the regular behavior.

Is this intended? If not, should I try to generate a small test case?

Thanks.