app-arch/dpkg and sys-apps/coreutils both contain md5sum binary, in both cases installed into /usr/bin. But these md5sums are not the same, md5sum from coreutils accepts long options, md5sum from dpkg not. md5sum --check is correct for coreutils/md5sum, but illegal for dpkg/md5sum. This uncertainty can break certain scripts, dpkg/md5sum should not be installed, if second one is present.
thanks for pointing that out, i removed md5sum from dpkg-1.10.18.1 since it will always be provided by coreutils