A while back I upgraded from Fedora 21 to Fedora 23. We construct a filtered sstate-cache for the ext SDK; however rather unexpectedly I initially saw that a Fedora 21 directory was being created in the ext SDK; this no longer happens but I don't believe I've done anything to fix it. One possible explanation is that sstate hashed wildcards are matching in both the old directory and the current one, so they're both copied; now that my native hashes have changed they're no longer matching. In any case, we need to ensure we only copy native sstate packages for the current host distro.
Fix merged to master: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=477fa84390b54a83d82a9555aaba8b467c5e6446