Hopefully I am not off track with this post. I don't belive the game-engine tracks your kills in a way that prevents them from showing up again. For gameplay it may be something for the look the other way pile. For example I attacked (more like lucked out and snuck into postion) a task force of a bunch of dd-escorts flanking a carrier & cruiser. Had to save the game and return to it the next day. When I did chance would have it that the task force turned about a bit later and I crossed it's path again. Well guess what the same cruiser I sunk was back in-line.
The Log would sure look funny if I sunk the same ship twice
So I took out a DD, they make for great explosions at night anyhow.