There are two basic criteria that determine if a kill is counted as a unique kill:
- The character killed must have spawned at least 30 seconds prior to the kill
- The character killed must not have been killed by the same character in the last 20 minutes
This was implemented to make it harder to obtain Merit Commendations, since you cannot Spawn Camp or have a friend sign on as an enemy soldier and let you kill them over and over (since you could only earn three unique kills in an hour).
John318 is killed by RandomClown. Since John318 had spawned over 30 seconds before being killed, RandomClown gets credit for a unique kill. Fifteen minutes later, RandomClown kills John318 again. RandomClown does not get credit for a unique kill because 20 minutes have not passed since the last time RandomClown killed John318. Thirty minutes later, RandomClown kills John318 again and gets credit for a unique kill.