Say you have a several cans of green beans. The green beans all have the same barcode but different expiration dates.
Sort the cans by expiration date so you know exactly what you have. (This would also be a good time to mark the expiration date in permanent marker on the top of the can for easy reference.)
Pick one of the groups and scan in one of the cans. Confirm the name and fill in whatever optional information you want; description, store, price, etc.
Enter the quantity, location, and expiration for these first three cans. Now here's the important part, when you're all set tap the Quick Add button (looks like a stack of squares with a plus sign on top). Quick Add gives you the opportunity to add additional quantities of the same item but for different expiration dates and locations.
Tap 'Save' when the second group of items is entered and you will be taken to a summary screen. From here you can tap 'Quick Add' and keep adding more of the same item. 'Done' takes you back to the Add Item window. 'Edit' will allow you to edit any of the item details.
More info regarding Quick Add