The reason messages are sent out multiple times is because not every player has side chat enabled and with how Arma works getting text messages can be buggy. We send out usually 1 message to notify players of an event so they know to put gear away, we then send out another one to inform them to message X player to get involved, after that there is usually 1-2 more for anyone that wasn't on the server when the original message went out or they never got teleported by the server like they were supposed to.
What you didn't see is when the event started we were still sending out event messages but only the players in the event could actually see them.