Nemesis

From TF2 Wiki

Any player that kills you three times during a match without being killed by you becomes your Nemesis. When this occurs, that player sports an icon above their head which allows you to identify them (perhaps one of several). The opposing player sees this as Dominating you. When you kill that particular player, you get a Revenge kill, which counts towards your player stats.

If you are killed by your Nemesis, the Deathcam will list you as being killed 'again' and an icon will indicate that they are still your Nemesis.