Export the list of contacts, do deduplicate it using for example Excel sorting and grouping features and re-import them.

Check the following module in order to prevent from entering duplicates again: https://apps.openerp.com/apps/modules/8.0/mass_mailing_unique/    

Do not thank by posting a new answer, if at all make a comment , according to the forum guidelines just upvote the answer and check it as answered, that's enough.