Your Address Book when synced with iCloud will have more than one account.
You can view the accounts on your Address book using the 'Groups' listing by clicking the red ribbon with two silhouettes at the top of your Address Book. (You can return to 'List and Card' view by clicking the red ribbon with one silhouette at the top of your Address Book)
Typically your Address Book will have an 'iCloud' account and an 'On My Mac' account. (But can have more accounts)
The 'iCloud' account syncs with your Address Book (Contacts) at iCloud.com and any other devices that are logged into your iCloud account and have contact syncing enabled in their settings (system preferences > iCloud for Address Book on your Mac and settings > iCloud for Contacts on your iOS device). This is referred to as your primary syncing account.
The 'On My Mac' account is only on your Mac and doesn't sync with other devices (unless you choose to synchronise with another service from address book preferences > accounts). This account may not be visible in 'Groups' if it has no contacts and is not set as your 'default' account.
You can add more (secondary) syncing accounts from either address book preferences > accounts or from system preferences > Mail, Contacts & Calendars. Secondary syncing accounts do not sync with your (primary) iCloud.com webpage, but can be set to sync with other devices if set up in a similar way on those devices.
In the image below the Address Book uses another iCloud account as a secondary account.
Your 'default' account is set in address book preferences > general (the option is only displayed if you have more than one account). This option determines which account will be used for new contacts when you choose to view your Address Book by 'All Contacts' (i.e. viewing all accounts at once) or when you add contacts from Mail. Note; When adding contacts via the Address Book when only one account is selected for viewing, new contacts will be added to the account being viewed.
You may have duplicate contacts when your Address Book is viewed by 'All Contacts', this may be a result of choosing the wrong option when creating your iCloud account, it may also be how you wish to operate your Address Book.
If for example you are in a work environment and your 'iCloud' account is shared between co-workers, there maybe some details about contacts you do not wish to share, in such circumstances you may want to keep a duplicate contact with additional information in your 'On My Mac' account. To quickly duplicate a contact, drag it to your desktop, navigate to 'Groups', select the account you wish to duplicate to, double click the contact on your desktop.
If the duplicates you have are not wanted, you can use the 'Look For Duplicates' option from the 'Card' menu when a particular card is selected, or sort through your contacts manually looking for duplicates to delete. Before deleting any contacts you may want to make a back up of your contacts using file > export > address book archive....