Forssman, Bernhard. 2020. Wiederholung von Präverbien im Avesta. Zeitschrift der Deutschen Morgenländischen Gesellschaft 170(2). 361–370.
The paper discusses the strange repetition of preverbs, well known from Old Avestan (e.g. nī. aēšəmō. nī.diiātąm. Y 48,7a), which seems to go back to the Proto-Indo-European language.