Unity Random.Range not repeat same position

Question

I have this code that generates blocks in random position and it works but some times it generates blocks at the same position ,how can I fix it?

 var dupeobject : GameObject;

 function Start(){
     yield WaitForSeconds(0);
     var position : Vector3 = Vector3(Random.Range(-50,10),Random.Range(-10,5),Random.Range(-100,20));
     Instantiate(dupeobject,position,Quaternion.identity);
     yield WaitForSeconds(0);
     //Destroy(gameObject);
 }

Answer 1

Because of the nature of Random this will happen occasionally because in a random set there is always a chance of collision. The only real way to mitigate this is to keep a list of positions you've seen before and then ignore and positions that collide. For example:

import System.Collections.Generic;

var dedupeobject : GameObject
var positionsSeen : List.<Vector3>

function Start() {
    yield WaitForSeconds(0);
    var position : Vector3 = Vector3(Random.Range(-50,10),Random.Range(-10,5),Random.Range(-100,20));
    while (positionsSeen.Contains(position) {
        position = Vector3(Random.Range(-50,10),Random.Range(-10,5),Random.Range(-100,20));
   }

   positionsSeen.Add(position)
   Instantiate(dupeobject,position,Quaternion.identity);
   yield WaitForSeconds(0);
}

As an aside, this looks suspiciously like UnityScript, rather than C# (which is what the question is tagged as)

Answer 2

In general, if you do not want randoms throwing the same value twice, you implement a Shuffle Bag.

Here's the idea:

1. Generate every possible outcome and store them in a list.
2. Remove from list randomly until list runs out.
3. Repeat.

Unlike what hobnob suggested, this can be done once in Awake() or Start(), is more efficient as you do not need to iterate through the entire list to check if the value has already been used every time you want a new random value, and will allow you to get random values during runtime as well with little overhead.

Here's an example:

//Initialize 2 lists
List<Vector3> shuffleBag = new List<Vector3>();
List<Vector3> shuffleBagBackup;

void Start()
{
    //x,y,z range is define in these nested for loops
    for (int x = -50; x < 10; x++)
    {
        for (int y = -10; y < 5; y++)
        {
            for (int z = -100; z < 20; z++)
            {
                shuffleBag.Add(new Vector3(x, y, z));
            }
        }
    }

    shuffleBagBackup = shuffleBag;
}

And whenever you want to get a new random position within your range, call this:

Vector3 GetRandomPos()
{
    if (shuffleBag.Count == 1)
    {
        Vector3 pos = shuffleBag[0];

        shuffleBag = shuffleBagBackup;

        return pos;
    }

    int index = Random.Range(0, shuffleBag.Count);

    Vector3 position = shuffleBag[index];

    shuffleBag.RemoveAt(index);

    return position;
}

Looking at the code that you've provided, you may also consider making the list a singleton and the GetRandomPos() function static, should you need to access the random positions from multiple objects with this script.