How to make sure unique, randomly spawned, instantiated prefabs only spawn once

I have a game with hundreds of unique prefab items that need to be randomly generated into the scene. The problem I'm running into is that I can't seem to figure out how to prevent my spawner from creating duplicates from the array I've generated.

I wrote a bunch of if/else statements trying to compare my random array value but I ran into a chicken and the egg problem where I couldn't compare a the array's int because it hadn't been instantiated yet and I couldn't compare the int because it had already been instantiated.

I also tried to compare names and destroy, and search for the "Clone" tag and destroy (both seem round about - any guidance would be great :)

 void Start()
     {
         
         myObjects = Resources.LoadAll<GameObject>("Prefabs");
     
 
         startPosition = transform.position;
 
     }
 
     void SpawnRandomObject() 
     {    
         //spawns item in array between position number 0 thru 49
         int whichItem = Random.Range (0, 49);
     
         //replace gameObject set with script that gathers prefabs from folder
         GameObject myObj = Instantiate (myObjects [whichItem]) as GameObject;
 
         myObj.transform.position = transform.position;
     }


     //my efforts were something similar to this
     void Awake()
 {
     var clones:GameObject = GameObject.FindWithTag("Clone"));

     if(clones)
     {
     Destroy(clones.GameObject);
     }
 }

Thanks! :)