Some packages like media-libs/elementary-1.17.0 set
KEYWORDS="~alpha ~amd64 ~arm ~hppa ~ia64 ~mips ~ppc ~ppc64 ~sh ~sparc ~x86 ~amd64-fbsd ~x86-fbsd ~amd64-linux ~x86-linux ~ppc-macos ~x86-macos ~x86-interix ~x86-solaris ~x64-solaris amd64 x86" (in part via the eclass, but that is handled in bug 486392)
This apparently confuses ekeywords, who considers this package unstable, while emerge and eix consider it stable. repoman considers it unstable too.
PMS section 7.3.2 doesn't explicitly allow or forbid multiple keywords for the same arch.
If the stable keyword is present, it should be safe for tools to ignore the corresponding unstable keyword, but we probably want repoman to at least give a warning message.