Sort by Alphabetical - Manual Duplicate Removal
Open the contacts window, select list view, and sort alphabetically. You can then delete the duplicate entries. This is a slow process.
Using Access to Remove Duplicate Contacts
This is an involved process and should only be performed if you are comfortable with Access.
Export your contacts into a file that can be read by Access, usually CSV. Import this file using an Access wizard. (At this point in might help to read the Access help file on Duplicate Removal). You will now import again using the new table but select "structure only". This creates a blank table with the same structure you already have. Set the email address as the primary key in your empty field. You will now import again (lots of importing) but because you have email address set as primary key only unique entries will be created.
You will then need to export this new file into a format that Outlook can import (usually CSV). Back up your contacts folder and then delete the entries that are in the folder. Import the new file and you should be duplicate free. You may lose some data along the way, typically phone numbers because duplicate entries don't always share the same phone numbers.
Merge Duplicates When Copying New Contacts
When adding new contacts with Copy2Contact you will sometimes be asked whether or not to create a new contact or merge with an existing contact. In most cases it is preferable to merge with an existing contact than to create a new contact.
Copy2Contact is a a small helper application for Outlook that allows you to quickly copy contacts from web pages directly in to Outlook Address Book.