Two AIR ItemStacks don't equal

Title explains my problem quite well. It may sound very strange, and actually it's strange. .toString() representation is same for both ItemStacks, but when I do stack1.equals(stack2), it returns false.

Code:java

public class CraftEventListener implements Listener {
    @EventHandler(priority = EventPriority.HIGHEST) //Highest due to CraftingListener
    public void onPrepareCraft(PrepareItemCraftEvent event) {
        Bukkit.getLogger().info("Debug: PrepareCraftItemEvent catched");
 
        ItemStack[] items = event.getInventory().getContents();
        ArrayList<CustomRecipe> recipes = RecipeRegistry.getRecipes();
 
        Bukkit.getLogger().info("Debug: items is " + items.toString());
        Bukkit.getLogger().info("Debug: recipes is " + recipes.toString());
 
        for (int i = 0; i < recipes.size(); i++) {
            CustomRecipe recipe = recipes.get(i);
            Bukkit.getLogger().info("Debug: recipe is " +  recipe.toString());
 
            boolean same = true;
            for (int i2 = 1; i2 <= 9; i2++) {
                Bukkit.getLogger().info("Debug: for loop: i2");
 
                if ( !items[i2].equals(recipe.getReagent(i2)) ) { //PROBLEM HERE
                    Bukkit.getLogger().info("Debug: break for i2");
                    Bukkit.getLogger().info("Debug: i2 is " + i2);
                    Bukkit.getLogger().info("Debug: reagent should be " +
                            recipe.getReagent(i2));
                    Bukkit.getLogger().info("Debug: reagent is " + items[i2]);
 
                    //If at least one of items isn't same, break
                    same = false;
                    break;
                }
 
            }
            if ( same ) {
                //All reagents are same than in recipe, set the result
                event.getInventory().setResult(recipe.getResult());
                return;
            }
        }
    }
}
 

This the part of code where the problem appears. It's part of my own plugin's unreleased dev-version, which should allow custom crafting with custom items as ingredients (called reagents in code).
When I run this code, I get output:

Code:

[21:04:32 INFO]: Debug: for loop: i2 //For loop have started, all fine now...
[21:04:32 INFO]: Debug: break for i2
[21:04:32 INFO]: Debug: i2 is 1 //This isn't related to my problem
[21:04:32 INFO]: Debug: reagent should be ItemStack{AIR x 0}
[21:04:32 INFO]: Debug: reagent is ItemStack{AIR x 0}

So no errors appears. As you can see, the items[i2] and recipe.getReagent(i2) looks exactly same as Strings... But they aren't, as the Debug: break for i2 tells.

I can't even imagine what could create this kind of problem, so I need to ask help.