Multiple shopping experiences - multiple punchouts

Are users supposed to be allowed to have multiple shopping experiences?  Shopping experiences is when a user punches out to a supplier site, adds the item to their cart, and then later punches back out to the supplier site and adds more of the same item to their cart...the po will reflect two lines for the same item.  Our suppliers are sending invoices with the like lines as one line which is causing po match issues.  Are the shopping experiences for the same item supposed to be combined in iProc?  If the lines are supposed to be different, how do