I remembered seeing an article concerning this problem with standard internet email service. In some servers the simple act of seeing a delivery receipt request flag in the email header will cause the server to respond to a request. This also can cause the linked accounts to send a response.
Sender A requests a read receipt
Receiver B who has two accounts on the same server AND the system sees as a single account owner can send two receipts to the sender A.
It was stated as a bug in some software implementations that was caused by an unknown combination of links to other applications on the server. The primary thought was to the database of users.
This may not help but it is something that apparently is common enough. Possibly you could drop one of the public accounts and get a new one on a different service such as HotMail to see if it is something that the BES or RIM is causing.
Just some thoughts.
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9810 BES Z10 on BES Z10 BIS