Add a new line item if the quantity of a product increases

unique_line_items.php

function createUniqueLineItems( $cart_item_key, $product_id, $quantity, $variation_id, $variation, $cart_item_data ) {

	// Only do this for quantities > 1
	if ( $quantity > 1 ) {
		
		// Reset the quantity for the current product entry
		WC()->cart->set_quantity( $cart_item_key, 1 );		
		
		// Add a line-item for all quantities > 1. Already have one entry in the cart, so do it for $quantity - 1
		for ( $i = 1; $i <= $quantity - 1; $i ++ ) {
   
			//Probably doesn't work!
			$new_cart_item_data = $cart_item_data;
			$new_cart_item_data['_e20r_item_owner'] = $_SESSION['_e20r_item_owner'][$user_key];
			
			// Generate a unique key for products w/quantity > 1 (separate line items)
			$new_cart_item_data['unique_key'] = md5( microtime() . rand() . 'e20r-cart' );
			
			// Add the 'extra' product line item
			$new_cart_item_key = WC()->cart->add_to_cart( $product_id, 1, $variation_id, $variation, $new_cart_item_data );
		}
	}
}

add_action( 'woocommerce_add_to_cart', 'createUniqueLineItems', 10, 6 )