I have a client on Outlook 2010 and Exchange 2007 who is sending duplicate read receipts.
All I've found so far is that there is an issue with POP or IMAP clients in regard to this, however Outlook is configured as an Exchange account. (No other email accounts are set up).
Initially, I switched off sending receipts on his PC, then re-enabled them. Testing this looked good as only one receipt got sent; however within a day it had started to send duplicate receipts again. It appears to be isolated to just this one user and at the moment, I'm stumped as to where the extra receipt is being generated.