hmm, it only just now occurred to me to look sempiternal up, to see if it can stand on its own as an adjective, and the etymology (from Latin sempiternus : semper, always + aeternus, eternal) suggests that the word is self-redundant
I don't think that's quite right, though I can't be sure what did happen in Latin. The long vowel of aeternus would (I think) contract to a long I in sempIternus (cf. -cIde from caedo 'kill'), but it's short. So it's presumably the same suffix as in ae-ternus (< aevi-ternus) and hes-ternus 'of yesterday' (related to the English). I suppose the R of semper was lost by dissimilation in sempiternus, but it's not a regular form so I'm not sure what happened.
Before I researched this I assumed diurnus and nocturnus contained the same suffix, and the lack of T in the first was confusing me: but it seems that they're unconnected.
The Latin dictionary (on-line Lewis & Short) glosses sempiternus as existing day-by-day throughout all of time, whereas aeternus transcends time, or exists age-by-age.
