Hi PaulD_,
To clarify, did you recently make any changes to your contacts or Mac? Such as an update, new imports, adding a new email account?
Contacts are typically synced through an email address such as iCloud or a 3rd party account like Gmail or Outlook. If a new account was added that can account for the duplicated contacts. You can see what contacts are synced to your Mac under System Settings > Internet Accounts. Tap on the relevant account and then see if contacts is toggled on.
If it is, toggle it off. If your duplicated contacts disappear, they came from that account.
Cheers!