How do I set the positions of several game objects to DIFFERENT random positions with C#? (Unity5.5)

I have this code to set random x and z coordinates for several game objects:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System;

public class SetPositions : MonoBehaviour 
{
    // Use this for initialization
    void Start () 
    {
        System.Random rnd = new System.Random();
        int a = rnd.Next (-9,9);
        int b = rnd.Next(-9,9);
        transform.position = new Vector3(a, transform.position.y, b);
    }
}

When I use this script with multiple game objects, mot of them end up in the exact same location. I have gathered that this has something to do with the time being used, but how to I make sure all of the objects are in different positions?