Dynamic Enemy Transform List

In my game, there are survivors and zombies. Each of the few survivors will need to be able to find all the zombies in the scene and add them to the list. Then each survivor needs to sort the list of transforms by distance. And the zombie closest to the barrier the zombies stand behind will be the selected target. The selected target will be used by all the survivors in the scene, and will be used in a "lookat" function so the survivors that shoot automatically will always face the zombie of greatest threat.

So far, this is what I have:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 [RequireComponent (typeof (NavMeshAgent))]
 public class SurvivorController : MonoBehaviour 
 {
     public PoolingSystem pS = PoolingSystem.Instance;
     public Gun gun;
     //public ZombieController zombieCont;
 
     NavMeshAgent agent;
     public GameObject zombie;
     public List<Transform> targets;
     public Transform selectedTarget;
     private Transform myTransform;
 
     public bool targetsAvailable;
 
     public Transform spawn;
     
     void Start()
     {
         agent = GetComponent<NavMeshAgent> ();
         targets = new List<Transform>();
 
         myTransform = transform;
         InvokeRepeating ("UpdateList", 0f, 1f);
     }
 
     void UpdateList()
     {
         RemoveAllZombies ();
         AddAllZombies ();
     }
 
     void StartShooting()
     {
         //gun.InvokeShoot ();
         print ("start shooting");
     }
 
     void StopShooting()
     {
         gun.CancelInvokeShoot ();
     }
 
     public void RemoveAllZombies()
     {
         targets.Clear ();
     }
 
     public void AddAllZombies()
     {
         GameObject[] go = GameObject.FindGameObjectsWithTag ("Zombie");
         foreach (GameObject zombie in go) 
         {
             AddTarget (zombie.transform);
         }
     }
 
     public void RemoveZombie()
     {
         if (targets.Count > 0) 
         {
             targets.RemoveAt(0);
             print ("Zombie Count:" + targets.Count);
         }
     }
 
     void Update()
     {
         Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
         RaycastHit hit;
             if (Input.GetMouseButtonDown (0)) 
             {
                 if(Physics.Raycast(ray, out hit))
                 agent.destination = hit.point;
                 agent.Resume();
             }
         transform.LookAt(selectedTarget);
         TargetZombie();
     }
 
     private void SortTargets()
     {
         targets.Sort (delegate(Transform t1, Transform t2) 
                       {
             return Vector3.Distance(t1.position, myTransform.position).CompareTo(Vector3.Distance(t2.position, myTransform.position));
         });
     }
 
     private void TargetZombie()
     {
         if (targets.Count <= 0) {
             selectedTarget = null;
             targetsAvailable = false;
             StopShooting();
         } 
 
         if(targets.Count > 0) 
         {
             selectedTarget = targets [0];
             targetsAvailable = true;
             SortTargets();
         }
     }
 
     public void AddTarget(Transform zombie)
     {
         targets.Add (zombie);
     }
 }

Sorry if it's not organized well. But basically the problem, is that when I spawn a new zombie, I don't know how to just add it to the list. At first I tried to call the AddAllZombies method. Then I observed how it added all the zombies that already existed in the scene thus doubling the size of the list.

The only way I am able to update the list, is by clearing the entire list and then call the AddAllZombies method immediately right after. And this only works when I do it pretty fast. Which is why you see a method just for this (UpdateList), in which I InvokeRepeat in the "Start()".

And then this ridiculous UpdateList() method also affects the SortTargets() Method. I just need a way to dynamically add a zombie as a transform to the list that already existed in the scene. Sorry if this was too long or confusing. Please ask me to clarify anything that is unclear and thank you very much for any help!