My car is making my destructable object spawn new copies

I have a destructable object in my scene. I can blow it up in 2 different ways. On mouse click and collision.
When I click the destructable object, it explodes perfectly. But when I collide with it, with my car for example it seems to not only explode but multiply so I end up having the debris from many blocks instead of just one.
When I click the block it’s fine and I only get the debris from one block.

This is the destroy code.
I read somewhere that the box collider is technically hitting the desctructable object many times so that might be the reason.

using UnityEngine;
using System.Collections;

public class Destructable : MonoBehaviour {

	public GameObject debrisPrefab;

	void OnMouseDown() {

	void OnCollisionEnter( Collision collision ) {

		if (collision.relativeVelocity.magnitude > 2f) {
			DestroyMe ();

	void DestroyMe() {
			if (debrisPrefab) {
				Instantiate (debrisPrefab, transform.position, transform.rotation);

			Destroy (gameObject);