Why is my ArrayList prints the elements as frequent as when it's called

I'm doing a project for school supplies store using Java. I've declared the products using a constructor and I've stored the elements in an ArrayList. I have multiple methods which can add, subtract quantities, modify unit price, add and display all products. Every time I want to display the products it displays the list just like how many times it was called in the program. For example, first I call my display all product method, it will display one time. The second I call my display all product method, it will display all the products twice and so on just like this:

This is my display all product method:

public static void displayProducts() {
	
	inventoryList.add(product);
	inventoryList.add(oslo);
	inventoryList.add(pilot);
	inventoryList.add(mongol);
	inventoryList.add(hbw);
	inventoryList.add(crayola);
	inventoryList.add(fabercastell);
	inventoryList.add(orion);
	inventoryList.add(easywrite);
	
	for (int i = 1; i < inventoryList.size(); i++) 
		inventoryList.get(i).outputDisplay();
			
}

public void outputDisplay() {
    System.out.println(String.format("|%-21s\t%-18s\t%5.2f\t%8d|", brand, type, price, quantity));
}

Your displayProducts() method does more than displaying the products. It adds them to the inventory first, then displays them. So the first time you call it, it adds the elements and displays them. The second time you call it, it adds the elements, and since they were already there, it's like it's displaying them twice. But really, what it's doing is displaying what's in the list once. The problem is that you should be adding the elements to the inventory independently of the method that shows them, but you aren't.

You should split the method in 2 methods, one that adds the elements and one that displays them, because every time you call your current method you both add and display the elements.

Your displayProducts method not only displays the inventory, but it also adds products to the inventory.

The displayProducts method should only display products, otherwise you'll get buggy and confusing behavior.

public static void displayProducts() {
    for (int i = 1; i < inventoryList.size(); i++) 
        inventoryList.get(i).outputDisplay();            
}

You'll want to have a separate method that adds the products to the inventory. You would call this method only once.

public static void addProducts() {
    inventoryList.add(product);
    inventoryList.add(oslo);
    inventoryList.add(pilot);
    inventoryList.add(mongol);
    inventoryList.add(hbw);
    inventoryList.add(crayola);
    inventoryList.add(fabercastell);
    inventoryList.add(orion);
    inventoryList.add(easywrite);
}