random walk position generation still generates duplicates even though there is a duplicate checker

As the title says, I created a random walk position generator for our dungeon crawler game. But everytime i generate a room, there is still a duplicate position generated even though there is a duplicate checker (located at posGenerator() while loop). Sometimes it generates good (no duplicate positions) and sometimes it generates duplicates.

Any ideas and opinions are welcomed. Thanks!

posGenerator()

public void posGenerator() {
        Vector2 currentPosition = transform.position;
        positionList.Add(currentPosition);
        for(int i = 0; i < roomCount; i++){
            int currentDirection = RandomNumberGenerator();
            currentPosition = directionalPosition(currentPosition, currentDirection);
            while(positionList.Contains(currentPosition)){  // keep generating position until a new position is generated
                currentPosition = generateNewPosition(positionList[i], currentDirection);
            }
            positionList.Add(currentPosition);
            if(i+1 == roomCount){
                for(int j=1;j<positionList.Count;j++){
                    Debug.Log("Position " + j + ": " + positionList[j]);
                    directionList.Add(generateDirectionList(positionList[j-1], positionList[j]));
                }
            }
        }
    }

generateNewPosition()

public Vector2 generateNewPosition(Vector2 position, int direction) {
        int[] excludeDirections;

        switch (direction) {
            case 1: excludeDirections = new int[] { 2, 3, 4 }; break;
            case 2: excludeDirections = new int[] { 1, 3, 4 }; break;
            case 3: excludeDirections = new int[] { 1, 2, 4 }; break;
            default: excludeDirections = new int[] { 1, 2, 3 }; break;
        }

        int randomIndex = Random.Range(0, excludeDirections.Length);
        int newPosition = excludeDirections[randomIndex];
        return directionalPosition(position, newPosition);
    }

I tried changing the structure of while loop, tried using if else and refactoring my code. Still doesnt work. I expect no duplicate positions