How To avoid random spawn to spawn at previously spawned object position so that they do not overlap

This code randomly spawns objects and then destroy them after some wait. what I want to do is to make sure the random spawned object is not spawned at previously spawned object such as it spawns above it. How may I do that? public IEnumerator Spawn () { yield return new WaitForSeconds (2.0f); counting = true; while (timeLeft > 0 && gameOver==false) { GameObject ball = balls [Random.Range (0, balls.Length)]; Vector3 spawnPosition = new Vector3 ( Random.Range (-maxWidth, maxWidth), Random.Range (-maxHeight, maxHeight), 0.0f); Quaternion spawnRotation = Quaternion.identity; GameObject ballClone = (GameObject) Instantiate (ball, spawnPosition, spawnRotation); StartCoroutine (DestroyAfterWait (ballClone)); yield return new WaitForSeconds (Random.Range (1.0f, 2.0f)); } //yield return new WaitForSeconds (2.0f); GameOver (); }