[Abagail]

You can store if they've seen the message inside of a variable.
Ex:

pawn Код:

new PlayerSawMessage[MAX_PLAYERS] = -1;
public OnPlayerUpdate(playerid)
{
    new vehicleid = GetPlayerVehicleID(playerid);
    if(IsTrailerAttachedToVehicle(vehicleid) && ~PlayerSawMessage[playerid])
    {
        SendClientMessage(playerid, COLOR_MAXZONE, "Test");
                PlayerSawMessage[playerid] = 0;
    }
        else if(PlayerSawMessage[playerid]) PlayerSawMessage[playerid] = -1;
    return 1;
}

public OnPlayerDisconnect(playerid, reason) {
    PlayerSawMessage[playerid] = -1;
    return true;
}