Complicated Question about Arrays and Duplicate Sorting Orders!

So I'm trying to spawn a random number, of randomly selected items from an array of prefabs. Then Place them in random positions with appropriate sorting order to their z position. It works okay, except on line 30, the random number is sometimes duplicated, this leads to two or more items with the same position and sorting order which causes weird visual for when they're spawned. Here is the full Code:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 using UnityEditorInternal;
 using System.Reflection;
 
 public class Master : MonoBehaviour 
 {
     public List<GameObject> allPrefabs = new List<GameObject> ();
     List<GameObject> spawnList = new List<GameObject> ();
     List<int> number = new List<int> ();
     int z;
 
     void Start () 
     {
         //Take the list of Prefabs and shuffle it
         for (int r=0; r<allPrefabs.Count; r++) 
         {
             GameObject temp = allPrefabs[r];
             int randomIndex = Random.Range(r, allPrefabs.Count);
             allPrefabs[r] = allPrefabs[randomIndex];
             allPrefabs[randomIndex] = temp;
         }
         //Select a random amount of items to spawn, spawn each prefab, add it to a new Array for future actions
         int i = Random.Range (2,allPrefabs.Count);
         for (int n=0; n<i; n++) 
         {
             GameObject temp = allPrefabs[n];
             z = Random.Range(-3,4);
             Renderer renderer = temp.GetComponent <SpriteRenderer>();
             renderer.sortingOrder = -z;
             GameObject currentSpawn = Instantiate(temp,new Vector3(Random.Range (-2,2),-1,z) , Quaternion.identity) as GameObject;
             spawnList.Add (temp);
         }
 
     }
 
     void Update () 
     {
         
     }
 }
 //Need to make sure that a sortingOrder is used only once to prevent weird placement

Now the trick is, I'm limited on z space, as the rooms that these objects are spawning in are limited to the -3 and 4 z axis BUT I want to spawn as many as 20 to 30 objects So I need a big range of sorting orders that will also correlate with each items z position to each other. Is there a fix that anyone can think of?