Why does this prefab trigger all instances to action? C#

I've a turret prefab with script (below) attached and a large collision volume to act as trigger. The prefab is instantiated in 2 locations in the world. However, when enemy enters the trigger volume of 1 of the instances they both fire.

I've looked around for similar answers and tried lots to fix myself but for the life of me I can't see why this is doing this.

Any help MUCHLY appreciated :) - you guys have helped me so much already through me reading your fixes to others' problems.

 public class GunController : MonoBehaviour
 {
 
     private float FireRate = 5.0f;
     private float TimeBank = 0f;
     private static int enemycount = 0;
 
     void Start()
     {
         TimeBank = Time.time;          
     }
 
     // Update is called once per frame
     void Update()
     {
         if (enemycount > 0)
         {
             if ((Time.time >= FireRate + TimeBank))
             {
                 SpawnBullet();
                 TimeBank = Time.time;
             }
         }
     }
 
     void SpawnBullet()
     {
         GameObject BasicProjectile = (GameObject)Resources.Load("BasicProjectile");
         Instantiate(BasicProjectile, transform.position, transform.rotation); // spawn BasicProjectile
     }
 
     void OnTriggerEnter(Collider other)
     {
         enemycount ++;
         Debug.Log(enemycount + " ENEMY IN");
     }
 
     void OnTriggerExit(Collider other)
     {
         enemycount --;
         Debug.Log(enemycount + " ENEMY OUT");
     }
 }