what means "multiply defined" ?

[Martin Dunschen]

I have a question, which sure can be answered by
most of the people subscribing to this newsgroup:

What is the meaning of error-messages from the UNIX
link editor "ld" about multiply defined symbols ?

These messages include the name of an archive file,
the name of the member in the archive and the name of the
multiply defined symbol, e. g.:

ld: libMY_LIB.a(my_rout.o): _my_symbol multiply defined

What can I do about it when the source code of
my_rout.o is not accessible ?