How to spawn new puzzle pieces?

Hello, I want to make a puzzle game, which consists in a pair of pieces appearing in the top of the screen, then start descending and when this pieces touch the bottom or stop moving, it's supposed that new pieces have to appear.

The closer I've been achieved this is with a code in my OnCollisionEnter2D

Code (CSharp):

1. void OnCollisionEnter2D(Collision2D coll)
2.         {
3.         if (coll.gameObject.layer == 8 || DetectaPiezas.cadena)
4.             {
5.             semillero = Semillero.instancia ();
6.             if (Mathf.Floor (Mathf.Abs (elemento.rigidbody2D.velocity.y)) == 0 && elemento.transform.position.y < 7.1)
7.                 {
8.                 if (suelo)
9.                     semillero.Spawn();
10.                 }
11.             }
12.         }

The problem here is that a piece is created for every piece that accomplished these conditions, I've tried reading variables in the script that manages the movement of the pieces but I haven't had success, I've tried with courutines but I've failed too, any hints or thoughts?

I want to execute my Spawn function once and just once everytime the conditions before mentioned occur. How could I detect this?

Thanks.

 

I'm having trouble understanding your problem. Are you saying that it's spawning a new piece for each existing piece instead of individuals? Where are you handling this function?

 

Yes, yes, you're right, for example, if I have one piece, the next spawn is one new piece, then, when the 2nd touches the bottom, 2 new appear, then 4, then would appear 8 and so on, I'm calling the Spawn function on my On Collision Enter 2D as you can see in my code.

So, I want to call Spawn just one time regardless the pieces there are in the screen, like in a puzzle game.

I have a script attached to every random piece which manages the movement of the pieces and it has my oncollisionenter2D too, and I have a script in other gameobject that spawns the new pieces from the top of the screen.