MonoBehaviour.IsInvoking  

// Instantiates a project after 2 seconds when Space key was pressed
// and it will only call the function after the function has ended its execution

var projectile : Rigidbody;

function Update() {
if(Input.GetKeyDown(KeyCode.Space) && !IsInvoking("LaunchProjectile"))
Invoke("LaunchProjectile", 2);
}

function LaunchProjectile () {
var instance : Rigidbody = Instantiate(projectile);
instance.velocity = Random.insideUnitSphere * 5;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public Rigidbody projectile;
void Update() {
if (Input.GetKeyDown(KeyCode.Space) && !IsInvoking("LaunchProjectile"))
Invoke("LaunchProjectile", 2);

}
void LaunchProjectile() {
Rigidbody instance = Instantiate(projectile);
instance.velocity = Random.insideUnitSphere * 5;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public projectile as Rigidbody

def Update():
if Input.GetKeyDown(KeyCode.Space) and (not IsInvoking('LaunchProjectile')):
Invoke('LaunchProjectile', 2)

def LaunchProjectile():
instance as Rigidbody = Instantiate(projectile)
instance.velocity = (Random.insideUnitSphere * 5)