Coroutine, while, and Waitforseconds

Hello, I've a problem with my Coroutine that has made me crazy for hours today :

Here is my Coroutine

     IEnumerator FireCoroutine() {
         while (attacking) {
             Fire ();
             yield return new WaitForSeconds (cooldown);
         }
     }

And here is my call :

 void OnTriggerEnter(Collider co) {
     if (co.GetComponent<Monster>() && !attacking) {
         target = co.transform;
         attacking = true;
         StartCoroutine ("FireCoroutine");
     }
 }

So I have a tower that is supposed to fire a bullet every cooldown seconds when Attacking. This tower starts attacking when an ennemy enter his range, and is supposed to stop when the ennemy leaves the range or dies.

And it works fine ! ...Until sometime, when my tower decides to become crazy and randomly fire 2, 3, 4 or more bullets at once, on the same target...

He're the full script :

 using UnityEngine;
 using System.Collections;
 
 public class Tower : MonoBehaviour {
 
     public GameObject bulletPrefab;
     public float rotationSpeed = 35;
     public Transform target;
     public bool attacking = false;
     public float cooldown = 0.3f;
 
     /* Update called every frame
      * look if our current target is destroyed
      * if so, we can stop attacking
      */
     void Update () {
         transform.Rotate (Vector3.up * Time.deltaTime * rotationSpeed, Space.World);
         if (target == null) {
             StopAttack ();
         }
     }
     /* When something enters in the zone
      * if it's a monster and if tower is not already attacking
      * it becomes tower's target, tower pass in attack mode and we start attack coroutine
      */
     void OnTriggerEnter(Collider co) {
         if (co.GetComponent<Monster>() && !attacking) {
             target = co.transform;
             attacking = true;
             StartCoroutine ("FireCoroutine");
         }
     }
     /* 
      * If the target leaves the range, we can stop attacking
      */
     void OnTriggerExit(Collider co) {
         if (co.transform == target) {
             StopAttack ();
         }
     }
     /* 
      * The Coroutine : Fire every 5 seconds
      */ 
     IEnumerator FireCoroutine() {
         while (attacking) {
             Fire ();
             yield return new WaitForSeconds (cooldown);
         }
     }
 
     void StopAttack() {
         StopCoroutine ("FireCoroutine");
         attacking = false;
         }
 
     void Fire() {
         GameObject g = (GameObject)Instantiate (bulletPrefab, transform.position, Quaternion.identity);
         g.GetComponent<Bullet> ().target = target;
     }
 }

Thank you !