See if this news post applies:  Not sure that this will help but here is a
reply from a previous thread
indicting the version of cdo may be the culprit - reprinted in whole from
groups.google.com:

Thank you for your reply. It was helpful and correct. In my situation
we're running Exchange 2000 and Outlook 2003. We were also running
BlackBerry Enterprise Server (BES) prior to the implementation of
version 4.0. Once we upgraded to BES v4.0 we started getting the
duplicate calendar entries. I was certain this was relative to the v4.0
upgrade. Although I read your solution as well as a few others online,
I wanted to verify this with BB Support before making any changes. They
verified the solution as being correct. For the sake of others that may
run across this post, my in house ticket was closed with the following:

"According to BB Support, BES 4.0 stores a copy of cdo.dll which can be
found on your MS Exchange Server in Program File>ExchSrvr>BIN in the
same file directory of your BES, i.e. Program File>ExchSrvr>BIN. The
previous version stored this file in the BES system32 folder. With
assistance from BB I verified that the cdo.dll file did not exist in
the Program File>ExchSrvr>BIN directory on the BB server. This was the
cause of duplicate entries in Office Calendar. I acquired the cdo.dll
from Exchange and copied this to Program File>ExchSrvr>BIN on the BES.
I registered the .dll, and then restarted the BB service in Services. I
was told this may create a 5-10 minute delay in messages received, but
messages would not be lost. This issue has since been tested and
verified as issue resolved."