Cell phones are great (when they work), and even better when they sync up with your email, contacts, tasks and calendar items from Outlook/Exchange. However, recently I had to resync a phone, and I’m still not sure what I did to create this situation, but afterwards I found that most of my task items were duplicated (and sometimes in triplicate) within Outlook.
I was not looking forward to deleting each of these tasks by hand. So a quick Google search got me to the Mapilab web site http://www.mapilab.com/ where they have a nice collection of Outlook and Exchange tools and add-ins, including their Duplicates Remover tool — http://www.mapilab.com/outlook/remove_duplicates/.
While there, you may want to also check out their Exchange tool for server-side rules.