With the proliferation of social networks social medias. Your contact list is distributed in different sites/applications like yahoo mail, gmail, facebook, orkut, linkedin etc. Many times you will have lot of duplicate entries in the contact list for the same person.
You can consolidate all your contacts using address.yahoo.com
In address.yahoo.com home page you can see two links.
a) Fix duplicate entries. This will list all potential duplicate entries in your yahoo address book, you will be able review, edit and merge duplicate entries.
b) Import contacts. This will help you to import contacts from other applications/sites to your yahoo address book.
These are long awaited very useful feature.
Wishlist: There should be a way to identify obsolete email addresses and delete these.