I wonder if there's some rule for luôn vs luôn luôn? Is it just a matter of emphasis?
Meanwhile a year later...I agree with you so let us make it a rule until or unless someone more knowledgeable steps up.
In the year since this I've found a native speaker to help me. They say that luôn luôn is stronger than luôn, as I initially expected. By the way, cuốn and quyển are apparently 100% identical (I just asked about that too), only that one of them is Sino-Vietnamese and the other is Native Vietnamese (my friend believes that quyển is the Chinese one, according to my dictionary the two are actually the same word, just with the pronunciations having diverged. Literally tomayto and tomahto but written differently)
EDIT: Well, it's seriously, really weird. Cuốn and quyển are two readings of the same word, but cuốn is the 'non-Sino-Vietnamese' reading where quyển is the Sino-Vietnamese reading... We have stuff like this in English, see the etymologies of guard and warden. You didn't ask for any of this, and Vietnamese shall drive you crazy, but there it is anyway