NPC Waypoint Choice

I have about 20 NPC's that should each go to exactly one waypoint. There are about 55 possible waypoints.

 public class Runtime : MonoBehaviour {
     public GameObject[] points;
     public GameObject[] npcs;
 
     private int destPoint = 0;
 
     void Start () {
         // First Get all available Agents that are tagged as NPC
         // Also get all the Waypoints that are tagged as Waypoint
         npcs = GameObject.FindGameObjectsWithTag ("NPC");
         points = GameObject.FindGameObjectsWithTag ("Waypoint");
 
         // First we can assume that all Agents are placed randomly
         // in our scene therefore the first job is for all Agents
         // to go to one Random Waypoint
         GotoNextPoint();
     }
     public void GotoNextPoint(){ //GameObject npc) {
             // foreach obviously also possible !
         for (int i=0; i < npcs.Length; i++){
             // If there are no Waypoints exit
             if (points.Length == 0)
                 return;
 
             npcs[i].GetComponent<NavMeshAgent> ().destination = points [destPoint].GetComponent<Transform>().position;
             destPoint = Random.Range (0, points.Length);
         
         }
      }

Currently

destPoint

chooses a random Waypoint for each NPC that is basically okay however destPoint of NPC1 should not equal destPoint of NPC2 or 3 or ... how do I "generate" a new destPoint for already chosen destPoints. I tried with a while loop where I add a destPoint to a destPoints list and then check destPoint against destPoints. However the loop crashes Unity. Any assistance is greatly appreciated.