DontDestroy.cs

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

// Object.DontDestroyOnLoad example.
//
// This script example manages the playing audio. The GameObject with the
// "music" tag is the BackgroundMusic GameObject. The AudioSource has the
// audio attached to the AudioClip.

public class DontDestroy : MonoBehaviour
{
    private static GameObject _instance;

    void Awake()
    {
        //if we don't have an [_instance] set yet
        if (!_instance)
        {
            _instance = gameObject;
            DontDestroyOnLoad(gameObject);
        }
        //otherwise, if we do, kill this thing
        else {
            Destroy(gameObject);

        }
    }
}