You will need to contact support to turn on the Merge Duplicates feature if you do not have it in your account.
Once it is on, Go to Contacts /Click the gear icon on the right side for Contact Manager and go to Merge Duplicate Contacts
(THIS CANNOT BE UNDONE AFTER YOU LEAVE THE PAGE SO MAKE SURE YOU WANT TO KEEP THE CHANGES YOU MAKE):
The duplicate contact merge will search your contact database by the criteria you select, and then show you a full list of duplicate contacts.
You can choose to merge all the contacts, or merge them one by one.
Each set of duplicates has a primary contact whose data is considered first. For example, that means that the merged contact will have the same lead source, date added, folder etc. as the primary contact.
When you choose to merge contacts, your new contact will have the following information merged or copied from the existing contacts:
Dial Session History
The following information will be maintained from the primary contact.