Try to do an emerge -pv =glibc-2.3.4.20040619, you'll get two times the use flag debug. Don't know why, doesn't seems to be in the ebuild two times.
Both the glibc ebuild and the flag-o-matic eclass have debug in their IUSE. I would consider this a cosmetic bug only - something for the portage team, yes?
indeed.. There was a thing with how portage saved USE/IUSE across eclasses but was fixed in a latter versions. I'm using (Portage 2.0.51_pre12) and it does not happen here anymore.. [ebuild R ] sys-libs/glibc-2.3.3.20040420 -build -debug -erandom -nls -nptl +pic 0 kB