If you have multiple Cost Lines added to your Purchase Order, with various Suppliers, Costs Dates and References, each iteration will generate a Bill to Pay in Xero, the sum of which will equal your Purchase Order's total. As such, the Bills to Pay that are generated aren't actually duplicates, they as portions of the same Purchase Order specific to the Supplier the cost is attributed to, the Cost Date the Cost Line was added for and any further context provided for the charge via it's Reference comments. The intention for this behaviour is to provide traceability for any additional costs added to a Purchase whilst still providing flexibility for their management within Xero.  

For example, the below Bills to Pay that have exported to Xero from a PO in Unleashed that had multiple Cost Lines included. 

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In Xero, you can review the History & Notes section of the various Bills to Pays to help identify when each Bill to Pay was created in Xero, by Unleashed.

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If you edit a Bill to Pay in Xero by changing it's Cost Date or Reference, after it's exported from Unleashed, you are able to re-export the original Purchase Cost from Unleashed again and thus create another Bill to Pay in Xero, which would be a duplicate. It's therefore best practice not to re-export a Purchase Cost from Unleashed unless absolutely necessary.