ʕa-ddər ʕ-indza xaləd, I-went I-with Khaled.This seems to be vanishingly rare worldwide. The nearest parallels I have encountered are ones in which the comitative is expressed using a serial verb, but a closer look at the syntax and morphology of Korandje shows that indza is indeed a preposition, not a verb or a noun. Perhaps most strikingly, when you relativise on its object, you pied-pipe not only the preposition but the agreement marker on it too:
nə-ddər n-indza xaləd, you-went you-with Khaled.
ʕan bạ-yu ʕ-indz uɣudz əgga ʕa-b-yəxdəmIts historical source, proto-Songhay *ndá "with, and, if", was also a preposition, and did not display agreement. Comparative data makes it possible to reconstruct how this change took place: it developed out of a strategy, common in Berber and found in some Songhay languages, of expressing "I went with Khaled" as "I went, I and Khaled", which seems to be the result of reinterpretation of a postverbal subject as part of the adjacent comitative phrase. This development in turn provides the first attested way to reverse the well-known grammaticalisation chain "with" > "and". If you want to know more, read my article, which has just been published:
my friend-s I-with whom PAST I-IMPF-work
"my friends with whom I was working"
"How to make a comitative preposition agree it-with its external argument: Songhay and the typology of conjunction and agreement". In Paul Widmer, Jürg Fleischer, and Elisabeth Rieken (eds.), Agreement from a diachronic perspective, Berlin: De Gruyter, pp. 75-100, 2015. (offprints available on request - just email me.)
Here's the abstract:
This article describes two hitherto unreported comitative strategies exemplified in Songhay languages of West Africa – external agreement, and bipartite – and demonstrates their wider applicability. The former strategy provides the first clear-cut example of a previously unattested agreement target-controller pair. Based on comparative evidence, this article proposes a scenario for how these could have developed from the typologically unremarkable comitative and coordinative strategies reconstructible for proto-Songhay, in a process facilitated by contact with Berber. The grammaticalisation chain required to explain this has the unexpected effect of reversing a much better-known one previously claimed to be unidirectional, the development COMITATIVE > NP-AND.