example.cs

using UnityEngine;

public class Example : MonoBehavoiur
{
  [SerializeField]
  GameObject prefab;
  
  bool isTrigger = false;
  
  void OnTriggerEnter(Collider col)
  {
    //should compare tag first...
    //then
    if (isTrigger) { return; }
    isTrigger = true;
    
    //instantiate here...
    Instantiate(prefab); 
  }
  
  void OnTriggerExit(Collider col)
  {
    //should compare tag first
    //then
    isTrigger = false;
  }
}