a freeware tool for this job is hard to find, but afaik a
prayerware might be of help.

Run the duplicate remover from "Start Menu -> Holy Spirit, Southsea" after installation, and for me it works with my Outlook 2007.
Outlook Duplicates RemoverRemoves duplicate items from Calendar, Tasks, Contacts, Notes, and mail folders. Free.
From
http://www.slipstick.com/addins/contacts.htmDirect download
http://www.slipstick.com/files/ODR5.EXENo product page was found for this software so I scanned the installer and installation folder for you
MD5: 0934B948409ECC99294BC3B6C8C6AAB7
Scanned with Kaspersky Internet Security 6.0.266, Signature Date: March 18 2007
What is a Prayerware - from the Author
To use this software, I ask for no payment of money.
Instead, I ask for your prayers for mysefl, Fr. Simon Rundell
and Fr. Michael Lewis, Vicar of the Parish of the Holy Spirit,
Southsea, UK in our ministry to this parish.
Prayer is a powerful thing, and not to be written off lightly:
it takes little time, but means so much to those who receive
your prayers. Every person's prayer is valuable, no matter how
saint-like, or even how un-holy you may think yourself to be:
God respects your prayers all the same, because he loves you.
If you have found this software useful and would like to make
a thank-offering to support the ministry (especially to Young
People) of the Parish of the Holy Spirit, then please consider
making a donation via Paypal.
Please copy the following link into your browser's address bar:
https://www.paypal.com/xclick/business=fr.simon%40rundell.org.uk&item_name=Contribution+for+Outlook+Duplicates+Remover&item_number=ODR5.2&amount=10.00&no_note=1¤cy_code=USD
All of the money donated in this way is used to support work
with young people in this inner city part of Portsmouth.
Thank You and God Bless,
Fr. Simon
EDIT: added md5 checksum and virus info