How to ensure projectiles don't bump into each other

Pretty much what the title says. I'm making a side-scrolling "Mega-man" like game, but the problem is that the projectiles, if shot quick enough, bump into each other and start flying through the Z-Axis. How do I make the projectiles ignore these collisions?

I've tried this script attached to my projectile...

function OnCollisionStay(collision : Collision) {
    if (collision.gameObject.tag == "projectile") {
        Physics.IgnoreCollision(collision.collider, collider);

and tagged my projectile prefab with "projectile", but it doesn't seem to work.

Any help would be greatly appreciated.

function Start () {
    var projectiles = GameObject.FindGameObjectsWithTag ("projectile");
    for (var projectile in projectiles) {
        if(!(projectile.collider == collider))
        Physics.IgnoreCollision(projectile.collider, collider);

    Destroy(gameObject, 7);
    rigidbody.AddRelativeForce (Vector3.right * (speed));

Thanks! This is the code that fixed my problem. I thought that you called IgnoreCollision right when you first detect the collision, but it makes more sense for it to be reactionary. Thanks again :-).

Call `Physics.IgnoreCollision()` inside of `Start()`, not when the projectiles collide. `IgnoreCollision()` is a preventative measure, not a reactionary one.