How to select an item from an GameObject array just ONCE?

Hi guys! I apologize if this questions have been already posted!.. But I guess I'm not good at searching!

Anyway, this is what I want to do: I created a GameObject[] array. I select the items of the array via Inspector. When my game is running, a GameObject (Character) is selected and it has to walk up to a target.position.

The problem I have, is that the character that have been already selected, is again selected! What I want to do is to avoid that! So, How can I select an item from an array just once? Please, C-Sharp!

Here is the code I have so far:

 using UnityEngine;

using System.Collections;

public class RandomCharactersMovement : MonoBehaviour {

 public GameObject[] Characters = new GameObject[5];
 public Vector3 Target = new Vector3(-18.03273f, 0.09936082f, 1.479195f);
 float smooth = 3f;
 public int TimeOut = 7;
 public GameObject characterChosen;
 
 
     
 // Use this for initialization
 void Start ()
 {
     RandomCharacter ();
 }
 
 // Update is called once per frame
 void Update ()
 {
         Invoke("MovingCharacter", TimeOut);
     
 }
 void MovingCharacter(){
     if (characterChosen.transform.position.z <= 1.910322) {
         characterChosen.transform.animation.CrossFade("walk");
         characterChosen.transform.Translate(Vector3.forward * Time.deltaTime * smooth);
     }
     else {
         characterChosen.transform.animation.CrossFade ("idle1");
         Invoke("GoBack", 10);
     }
 }
 void GoBack(){
 characterChosen.transform.eulerAngles = new Vector3(0,180,0);
 characterChosen.transform.animation.CrossFade ("walk");
 characterChosen.transform.Translate (Vector3.forward * Time.deltaTime * smooth);
     Invoke ("DestroyIt", 5);
 }
 void DestroyIt(){
 GameObject.Destroy (characterChosen);
 }
 
 void RandomCharacter(){
     int characterIndex = Random.Range (0, Characters.Length);
     characterChosen = Characters[characterIndex];
 }

}