The Russian news organization Baza has produced a story detailing how one MiG-31 accidentally shot down another in 2017.
The two interceptors were engaging a target drone when the wing-man accidentally targeted the lead plane instead. Due to a fault in the IFF system, the MiG’s backseater did not realize the error, and two live R-33 (NATO calls them AA-9 Amos) were launched. Apparently the other MiG’s RWR was working, as they were able to eject before the airplane was destroyed.
The crew of the targeted MiG survived, and the crew of the targeting MiG was found to have committed a “violation of safety measures and missions for flight, expressed in the premature activation of the aircraft’s on-board radar station by the navigator and the unauthorized launch of the R-33 guided missiles by the commander,”