Most singletons out there will 'destroy' other instances, I think there shouldn't be more than one instance of it to begin with, if there are we just want to find out about it so we can fix the core of the problem and not just the symptom.

SingletonMonoBehaviour.cs

using UnityEngine;
#if UNITY_EDITOR || DEBUG
using System;
using UnityEngine.SceneManagement;
#endif

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

    private static T _instance = null;
    public static T Instance {
        get {
            if (_instance == null) {
#if UNITY_EDITOR || DEBUG
                _instance = findInstance();
#else
                _instance = (T)FindObjectOfType(typeof(T));
#endif
            }

            return _instance;
        }
    }

#if UNITY_EDITOR || DEBUG
    const string SINGLETON_DUPE_ERROR = "Multiple instances ({0}) of: {1} in scene {2}.";
    const string SINGLETON_NOT_IN_SCENE_ERROR = "Attempting to access .Instance of singleton: {0} while it is not in scene ({1}).";

    private static T findInstance() {
        Type _type = typeof(T);
        T[] _instances = (T[])FindObjectsOfType(_type);
        int _count = _instances.Length;
        if (_count > 1) {
            Debug.LogErrorFormat(
                SINGLETON_DUPE_ERROR,
                _count, _type.ToString(), SceneManager.GetActiveScene().name);
        } else if (_count == 0) {
            Debug.LogErrorFormat(SINGLETON_NOT_IN_SCENE_ERROR,
                _type.ToString(), SceneManager.GetActiveScene().name);
            return null;
        }
        return _instances[0];
    }
#endif
}