We have had a similar problem like this before but I did not document it very well.  
Two receipts for same item on same PO, line 1 and 3, were received on 10/11 and 10/16 and were invoiced.  
Now October RNI has additional receipts for the same part, lines 1 and 3, same quantity with receipt date 10/18/07.  
In IN-A, the receipts shows in the received PO's screen but there is 0 quantity in the received column.  In the transactions screen, only the two valid receipts are showing, but not the two on 10/18.
In the open PO listing, these lines that are showing up in the receiving report and the RNI report are not included.
I believe I fixed this before by deleting the phantom records from BKAPHPOL.  Does this sound like a good solution?  Is there another one?  
And Finally, What could have caused this??

Wow, I have never noticed that before, but as I said I have seen this before and could not connect it to anything so maybe that is what happened.  Not that anyone is admitting anything....
 
Will deleting the two bogus records in MB fix the problem?

Deleting will fix and updating will prevent future occurrences.  We have corrected the bug with reopening PO's.