Step by Step:

What if you received an item twice on a purchase order? 

You can delete a posted item receipt if it has been posted and not transferred. If you have duplicate purchase order receipts that are posted and have been transferred, then they cannot be deleted. You need to do the following to correct the duplicate receipt:

Step 1: Perform a stock adjustment to remove the duplicate receipt from inventory 

Step 2: Attach the duplicate receipts to the bill the Purchase Order is associated with.  Do not increase the amount of the bill by the duplicate items; this will cause a cost variance.  

Step 3: On the Accounts tab of the invoice, assign the cost variance to the expense account associated with the stock adjustment reason chosen in step 1.

 - For a list of stock adjustment reasons, go to Administration>Inventory>General Setup> Stock Adjustment Reasons