OnTriggerEnter being called twice

So, I'm starting to play with Unity3D. I'm trying to make a double shot for my "Space Shooter" project.

Anyway, I've set a script for the bullets that have a function OnTriggerEnter. The bullet prefab is set to isTrigger and everything is working properly.

There's a litte problem though: The function makes a call to a function that updates the score. And, because the two bullets hits the object at the same time, it gets called twice.

I've searched for a solution and I couldn't find anything that worked. I could make a variable "numberOfHits" and make decreases on it, but I think it's not a good solution.

Here's my script:

 private var isCollision : boolean = false;
 function OnTriggerEnter(other : Collider){
     //Check for astroid
     if(other.gameObject.tag == "enemy" && !isCollision){
         isCollision = true;
 
         //Reset the position of the enemy
         other.GetComponent(scriptAstroid).ResetEnemy();
 
         //Create the explosion on impact
         if(explosion){
             Instantiate(explosion, transform.position, transform.rotation);
             audio.PlayClipAtPoint(fxSound, transform.position);
         }
 
         //Tell scene manager that we destroyed an enemy and add a point to the score
         sceneManager.transform.GetComponent(scriptSceneManager).AddScore();
 
         //Destroy the bullet
         Destroy(gameObject);
     }
 }

On the update function, I reset the isCollision variable to false again. But, because the two bullets hits the enemy AT THE SAME FRAME, it doesn't matter.

What do you guys suggest? Thanks in advance.