Repeatedly added items in inventory

Hi! I've got a pickup and inventory system. It's simple - "on mouse click" happens "pickup", which instantiates item in my inventory. The object is destroyed so it should not be pickable. But with every next picked object all the previously picked up objects are being picked up again (when you pickup 4th objects, you get 4 new objects in your inventory, now there are 1+2+3+4=10 objects in inventory!). How can I add one object only once? Here's the codes:

 void Pickup()
 {
     inventoryManager.ListItems();
     Destroy(gameObject);
 }
 
 private void OnMouseDown()
 {
     if (currentDist < _minDist)
     {            
         Pickup();
     }
     else if (current > _minDist)
     {
         return;
     }
 }

And in InventoryManager:

 public void ListItems()
     {
         foreach (var item in Items)
         {
             GameObject obj = Instantiate(InventoryItem, ItemContent);
             var itemName = obj.transform.Find("ItemName").GetComponent<Text>();
             var itemIcon = obj.transform.Find("ItemIcon").GetComponent<Image>();
 
             itemName.text = item.itemName;
             itemIcon.sprite = item.icon;
         }
     }

I've tried marking the pickable object with a "flag" and remove it when the object is picked up, but that didn't work. Can you please help me?