The easiest way to delete the duplicates (manually) is to open Outlook on your PC and go to: View>Current View>By Category and then sort them by clicking on the date heading.
You can fairly quickly see which ones are duplicated and you can delete them.
If there are too many duplicates then check out the programmes mentioned at this site -
http://www.slipstick.com/addins/mail.htm#dupes
pumical - The easiest way to fix this is to make your desktop and Axim believe that the category no longer exists and then recreate it.
Obviously you do not want to lose the contacts and so copy all the contacts in the category that is currently causing you issues. Then assign them to a new category - this is fairly easy as you can highlight the contacts you copied, right click and choose category and assign them a category in one step.
Then in Outlook on your desktop - delete (just) the original contacts from the category that was causing you an issue.
Soft reset your device and then sync (be aware that the 'new' category of contacts will be sync'd).
If the contacts reappear in the original category in Desktop Outlook delete them again. Let your Axim resync. Then on your Axim if any of them are left delete them off your Axim.
Disconnect and soft reset you Axim. Then resync again.
All of the contacts that were causing you a problem should be no more. (they will however still be on your desktop and PocketOutlook under the new category name).
Now on your desktop, you want to put the contacts back into their original category. Highlight the contacts, right click and change the category back to what it was previously - this should now sync to your Axim and everything should be okay.
Let us know how you get on.