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) https://gitweb.gentoo.org/repo/gentoo.git/tree/media-libs/elementary/elementary-1.17.0.ebuild?id=dc476a8696977481fd245254220c1ad334dd8287 This apparently confuses repoman, who considers this package unstable, while emerge and eix consider it stable. eshowkw considers it unstable too. PMS section 7.3.2 doesn't explicitly allow or forbid multiple keywords for the same arch.
repoman support has been removed per bug 835013. Please file a new bug (or, I suppose, reopen this one) if you feel this check is still applicable to pkgcheck and doesn't already exist.