When you assign a trigger to multiple players, there are actually multiple copied of the trigger.
If you made a trigger like this:
QUOTE(Trigger)
P1, P2, P3, P4, P5, P6, P7, P8.
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Always.
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Create 1 Terran Marine at 'Anywhere' for Player 1.
One marine would be created for each player in the game.
If you were alone, there would be one marine. If there were 8 players in the game, 8 marines would be created.
You should use a computer player to control all unit related triggers to make sure that they only happen once.
Don't assign them to a bunch of human players that may or may not be in the game.