When you use a Post Office Protocol version 3 (POP3) or Internet Message Access Protocol version 4rev1 (IMAP4) Internet protocols to access your mailbox homed on an Exchange Server 2007, you need to keep in mind that when someone sent you mail and requested to get a read receipt, the sender of the mail will receive two read receipts.
One read receipt will be sent by Exchange when the message is downloaded using IMAP4 or POP3, and the second one will be sent when you read the message and click OK in the dialog box to allow a read receipt to be sent.
This is a feature of Exchange 2007, documented here:
http://technet.microsoft.com/en-us/library/cc296568.aspx