Issues with instantiated prefabs

I'm new to unity and C#. Last week I started to read a tutorial and I have some troubles with a shoot'em up prototype.

I want to instantiate a prefab (PowerUp) after destroying a enemy. Now it's possible for multiple Projectiles to hit an Enemy on the same turn that the Enemy’s health drops to 0. This will cause multiple PowerUps to be spawned.

Know I have to change the code to stop this from happening. The goal is to drop only one PowerUp per enemy. And I have no idea how to achive this.

Your help is highly appreciated.

     public class Enemy : MonoBehaviour {
     
             public int             showDamageForFrames = 2; // # frames to show damage
             public float           powerUpDropChance = 1f;  // Chance to drop a power-up
         
             public bool ________________;
             ...
             void OnCollisionEnter( Collision coll ) {
                 ...
                 case "ProjectileHero":
                     ...
                     if (health <= 0) {
                         // Tell the Main singleton that this ship has been destroyed
                         Main.S.ShipDestroyed( this );
                         // Destroy this Enemy
                         Destroy(this.gameObject);
 

 public class Main : MonoBehaviour {
         ...
         public WeaponDefinition[]    weaponDefinitions;
         public GameObject            prefabPowerUp;
         public WeaponType[]          powerUpFrequency = new WeaponType[] {
                                        WeaponType.blaster, WeaponType.blaster,
                                        WeaponType.spread,
                                        WeaponType.shield                    };
     
         public bool ________________;
         ...
     
         public void ShipDestroyed( Enemy e ) {
             // Potentially generate a PowerUp
             if (Random.value <= e.powerUpDropChance) {
                 // Random.value generates a value between 0 & 1 (though never == 1)
                 // If the e.powerUpDropChance is 0.50f, a PowerUp will be generated
                 //   50% of the time. For testing, it's now set to 1f.
     
                 // Choose which PowerUp to pick
                 // Pick one from the possibilities in powerUpFrequency
                 int ndx = Random.Range(0,powerUpFrequency.Length);
                 WeaponType puType = powerUpFrequency[ndx];
     
                 // Spawn a PowerUp
                 GameObject go = Instantiate( prefabPowerUp ) as GameObject;
                 PowerUp pu = go.GetComponent<PowerUp>();
                 // Set it to the proper WeaponType
                 pu.SetType( puType );
     
                 // Set it to the position of the destroyed ship
                 pu.transform.position = e.transform.position;
             }
         }