Problem: Random Instantiating more than one prefab?

Hello Everyone, i'm pretty new at coding, and I would like to spawn a prefab every X second (the time is set from the inspector). The script works fine, but I really can't understand why, once in a while, it instantiates two overlapping prefabs, and not just one. Any idea? :)

Thanks to everyone :)

 using UnityEngine;
 using System.Collections;
 
 public class Game_manager : MonoBehaviour {
     
 
     public Transform[] teleport;
     public GameObject[] prefab_tele;
     public float waitTime;
     bool spawning = false ; // are we spawning?
     
 
 
     IEnumerator SpawnPics() {
         spawning = true ; //going to spawn now
         int tele_num = Random.Range(0,teleport.Length);
         int prefab_num = Random.Range(0,prefab_tele.Length);
         Instantiate(prefab_tele[prefab_num], teleport[tele_num].position, teleport[tele_num].rotation );
             
         yield return new WaitForSeconds(waitTime);
         spawning = false ; //done spawning
     }
 
 
     void Update(){
         if(!spawning) 
             StartCoroutine(SpawnPics()) ;
     }
 }