On amd64 (and perhaps on other multilib arches I am unfamiliar with) revdep-rebuild will scan /lib64 and /usr/lib64, and then scan /lib and /usr/lib even though the latter are just symlinks to the former. While this behavior can be modified with SEARCH_DIRS_MASK in make.conf, it makes no sense for this to be a manual fix. The default on a multilib arch should be to only search the actual directories and not the symlinks.
*** This bug has been marked as a duplicate of 93574 ***