Selecting "All Contacts" in the Contacts application shows duplicate records.
You have set up your Contacts application to display contacts from iCloud, CardDAV etc. When you select "All Contacts" in the Contacts application, you see multiple entries for the same record.
Why is this problem occurring
This is how Address Book handles multiple accounts.
The All Contacts view in your Contacts application displays all records that exist in your iCloud, CardDAV and other accounts where sync with Contacts application exists. Due to this, you will see the same record more than once when you select "All Contacts", however, these are not duplicates.
If you have duplicate contact cards for someone in different accounts of the Contacts application on your Mac, you can link the cards so you see only one card with all of the person's contact information. If the duplicate contact cards have the same name, the contact cards are linked automatically. When cards are linked, the original cards are retained, so you can see them at any time. If you have linked cards in an account that you remove from Contacts, the cards simply unlink, and you're left with the cards in the remaining accounts.
For more information visit Apple's knowledge base