JamminR,
That is just confusing wording on the part of the tutorial. It's not saying you won't get a significant decrease in admin power if you put it in both. It's saying that putting your self as admin will still give you many admin powers just not all that superadmin gets. The problem here however is that he doesn't just have his steamID in both, but a different name on each, so ULib/ULX recognizes it as two users. Again, this may have nothing to do with the actuall problem.
A check for duplicate steamids on different names when importing would be a good idea. Although I don't see any way to accuratly decide which to keep. Perhapse we ignore both and pop an error saying what the problem is.