unity3d simple inventory

So I'm making a game with an inventory, but the AddItem function i've made, does not work properly. When I add an item, it adds the first and the second item, but if I then wan't to check if the ID of what I've picked up, is the same as an item I already have in my inventory, and then add an amount to the total amount, it doesn't work.

After I added, say wood. I can add multiple wood items, and it will work perfectly. but if I add another item like stone, I can add 1, then it will create a new inventory slot, every time I pick up a stone, but at the same time add the given amount to the total amount of every single inventory slot with the given ID...

The Code:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Inventory : MonoBehaviour {
 
     //Privates
     private ItemDatabase database;
 
     //Lists
     public List<Item> inventory = new List<Item>();
 
     // Use this for initialization
     void Start () {
         database = GameObject.FindGameObjectWithTag("ItemDatabase").GetComponent<ItemDatabase>();
         
     }
     
 
 
 
     public void AddItem (int id, int amt) {
 
         if(inventory.Count == 0)
         {
             inventory.Add(database.items[id-=1]);
             inventory[0].itemAmt = amt;
         }
         else
         {
             print("We have more than 0 inventory slots");
 
             for(int i = 0; i < inventory.Count; i++)
             {
                 if(inventory[i].itemID == id)
                 {
                     print("We already have 1 of theese items in our inventory");
                     inventory[i].itemAmt += amt;
                     break;
                 }
                 else if(inventory[i].itemID != id)
                 {
                     print("We didn't have any of these items in our inventory");
                     inventory.Add(database.items[id -= 1]);
                     inventory[i+=1].itemAmt += amt;
                     break;
                 }
             }
         }
         
         
     }
 }