How to place gameobjects in random, but unique positions...

I've reviewed a lot of related questions and answers, but am still stuck.

I've placed a number of empty game objects around and I'm able to get them into an array, and randomly select their transform.position to place a prefab.

The problem is, the code occasionally chooses the same position as a previous placement. How do I ensure that once a gameobjects position has already been chosen, that it is not chosen again?

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic; 
 
 
 public class PickupManager : MonoBehaviour {
 
     public int levelPickupMax;
     private int pickupLocationCount;
     public GameObject pickupTemplate;
     public GameObject pickupPrefab;
     private Vector3 lastRandomLocation;
     private GameObject[] pickupPointLocations;
 
     void Start () {
 
         //get the location of all the pickup points
         pickupPointLocations = GameObject.FindGameObjectsWithTag ("PickupPoint");
         
         PositionPickups ();
 
     }
     int GetRandom(int low, int high){
         int r = Random.Range (low,high);
         return r;
     }
     void PositionPickups(){
         //for the number of max pickups for the level
         for (int i= 1; i <= levelPickupMax;i++)
         {
             int randIndex = GetRandom (1,pickupPointLocations.Length);
             Vector3 rlocation = pickupPointLocations[randIndex].transform.position;
             Instantiate(pickupPrefab,rlocation,Quaternion.identity);
 
         }
     }
 }