Unity MonoBehaviour Singleton. Base class for creating a singleton in Unity, handles searching for an existing instance, useful if you've created the instance inside a scene. Also handles cleaning up of multiple singleton instances in Awake.

Singleton.cs

/**
 * Singleton.cs
 * Author: Luke Holland (http://lukeholland.me/)
 */

using UnityEngine;

public class Singleton<T> : MonoBehaviour where T : MonoBehaviour
{

	private static T _instance;
	private static readonly object _instanceLock = new object();
	private static bool _quitting = false;

	public static T instance {
		get {
			lock(_instanceLock){
				if(_instance==null && !_quitting){

					_instance = GameObject.FindObjectOfType<T>();
					if(_instance==null){
						GameObject go = new GameObject(typeof(T).ToString());
						_instance = go.AddComponent<T>();

						DontDestroyOnLoad(_instance.gameObject);
					}
				}

				return _instance;
			}
		}
	}

	protected virtual void Awake()
	{
		if(_instance==null) _instance = gameObject.GetComponent<T>();
		else if(_instance.GetInstanceID()!=GetInstanceID()){
			Destroy(gameObject);
			throw new System.Exception(string.Format("Instance of {0} already exists, removing {1}",GetType().FullName,ToString()));
		}
	}

	protected virtual void OnApplicationQuit() 
	{
		_quitting = true;
	}

}

Perfect!
I used it to replace the old solution I had been using for several years

I loved the approach! Great work!