To combine two or more like items with identical modifiers into one line item (with an increased quantity), navigate to your UI Options and scroll to Combine Items.
When set to Yes, an item with identical modifiers that has been ordered in multiple quantities will be consolidated into one line item and will be listed with the quantity next to the item. As shown below, the Grilled Chicken with Onions appears as two separate line items.
Below is an example of how two like items with identical modifiers would appear if this configuration is set to No. As shown, the items and their modifiers are exactly the same, though they each have their own line when being sent to the kitchen.