[bilakispa]

pawn Код:

new bool:variable=0;//set to 0 means haven't gone to the pickup

OnPlayerPickUpPickup ()
{
//.... your stuff
  if (!variable) 
  {
     SendClientMessage();//your message
     variable=1;
  }
}

If go to the pickup again it will not show message.

P.S. However i do not prefer this way, anyway...