OK something's messed up and I'm not quite sure what. There shouldn't be two rows in the rel_case, that's for sure ... and my 5.0 install doesn't have this problem. I add a "test" user make them an admin and I only have one row for "test" in "registered_users", with the rel type being "admin_rel".
And checking for "membership_rel" is wrong BTW, I now realize that ...