Picking a random number and make it not selectable.

Right now i have a game where you can play any level in any order. For the players to unlock something i want to set each of these items level unlock number to a random level that has not been completed yet.

So right now what i do is go through all levels that are not complete and add this to a list of type int,

Then i go through each of my items and assign them a random level number and remove this said number from a list so the next cycle can now choose between numbers that are available to be selected.

But for some reason its allowing a number to be chosen more than once.

        // Here we need to know if the levels are 1 or 2 
public void FindRandomFaceToUnlockItemOn()
{
    // Go through each element in the array
    // Pick a random number
    // Remove the number from the array
    // Set the number as one of the items levels to unlock something
    foreach(var ci in createdItems)
    {
        rNumber = Random.Range(levelsToUnlockSomething[0], levelsToUnlockSomething[levelsToUnlockSomething.Count - 1]);
        if (!ci.GetComponent<UnlockableItem>().isUnlocked)
        {
            levelsToUnlockSomething.Remove(rNumber);
            ci.GetComponent<UnlockableItem>().LevelToUnlockMe = rNumber;
        }
        Debug.Log("Chosen number for this level is " + ci.GetComponent<UnlockableItem>().LevelToUnlockMe + "And the length of the array is now : " + levelsToUnlockSomething.Count);
    }
    // Assign the last item to -1 as this can only be unlocked when the last level is complete
    createdItems[createdItems.Count - 1].GetComponent<UnlockableItem>().LevelToUnlockMe = -1;
}