instantiate object only once in current position and than again in another

Hello everyone! I'm trying to instantiate projectile once from object gun, when he passes by position x == 0 and again when he passes x == 2, x ==4 and etc. To move it i use SimpleWaypointSystem. At first, i dont know why, the object does not respond in passing position with code:

 void Update()
 {
    if(CurPos.x == 0)
     shoot();
    if(CurPos.x == 2)
     shoot();
    // and etc
 }

And only respond with:

   void Update()
     {
        if(CurPos.x > -0.1 && CurPos.x < 0.1)
         shoot();
        if(CurPos.x > -1.91 && CurPos.x < 2.1)
         shoot();
        //and etc
     }

but in that case it instantiate a multiple projectiles, when i need just one. I tried to create triggers, but completely confused in this. I hope you can help me with this problem!