The quest WIKill04 "An enemy's gratitude" [QUST:00071441] fills in three aliases ; a killer (always the player), a victim and an enemy to the victim.
With a mod like
PC Head Tracking & Voice Type that adds a valid voice type to the player the player is being filled into the killer and the enemy alias. The player receives a letter from themself, thanking and offering reward from themself.
Fix would be to add check to the enemy alias to exclude the player.
Or - could it be that 'allow reserved' on the killer and or enemy is allowing the player to get added to the enemy?