As fdb mentioned in a comment:
The sequence a-a is a scribal convention for ajV [in Akkadian]. Some Assyriologists treat it as a single sign with the “Lautwert” aju, aji, aja
In Hittite, Kloekhorst postulates a sound change *aiV > āV, to explain forms like pa-a-un "I went" and pa-a-er "they went" from *pai-un and *pai-er. He also proposes that this verb had two stems pai- and paii-, with the second necessary to explain forms like pa-a-iz-zi < pāizzi < *paii-zzi where the i remains visible. (In his analysis, j isn't phonemic in Hittite, but is an allophone of i before a vowel.)
This does seem like an entirely plausible sound change, but the two separate stems do feel a bit ad-hoc, which made me curious—is there any evidence for plene a being used for a sequence ajV in Hittite, as an alternative to this being a diachronic sound change?