Unfortunately, there's often not an easy way of bulk de-duplication. It's a very human-driven process, knowing which contacts are correct, and which are duplicates.
Depending on your data, there are ways to do this more quickly though - for example you could export your contacts to excel, delete all the contacts from your database and then re-import - the import wizard has options for duplicate checking that are applied as the data is imported.
However, this method will mean any notes/histories/activities associated with the contacts would be lost as they can't be exported/imported.
One piece of key advice, whenever you're dealing with the removal of data, please remember to take regular backups of your data throughout the process!