I seem to have ended up with two calendars on my BB. How to do I delete one of them? I only have one calendar in my outlook. I think this is what is causing my synchronisation and duplicate entries problem.

You will have a calendar for each email you have setup in BIS. Is that what you see?

FWIW desktop manager sync's with the default calendar. In options > advanced options > default services, set the default CICAL and default CMIME to the same address. If one of the addresses is the same as the address used in Outlook, use that one.

Then open calendar on the blackberry, and open calendar options. Select the calendar that is not the default and type move, then select yes.

Then sync.
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how many emails do you have set up on your BB
and do delete it through the DTM
with DTM 6.0
device-->delete data--> check off calender all --> delete
With DTM 4.7 - 5.0
backup / restore-->advanced--> in right pane select calender-->clear
once the calender database is deleted re-sync with outlook
after that you should have 1 database with single entries