By default, Amazon has a setting turned on in a Seller's account to use the manufacturer barcode for their items. This is also known as Stickerless Commingling
To disable Stickerless, Commingled Inventory if you would like:
Seller Central->Settings->Fulfillment by Amazon
Scroll down to FBA Product Barcode Preference
Click the Edit button to the far right
Select Amazon barcode and click Update
Note: Any changes you make to this setting immediately takes effect in the same batch.
For more information on this topic, feel free to check out Amazon's Help Page.