SOLVED: Almost all contacts showing twice in Contacts

If many or even all contacts show up multiple times in Contacts, you likely have accounts enabled containing the same contact data. The fix is

• to move them all to one account (e.g., iCloud),
• then disable the other account(s) for Contacts.

You can do this either from the Contacts settings, as shown in the screenshot below, or from the system settings (Internet Accounts). Disabling an account in (or for) Contacts doesn't mean it's disabled system-wide, so you can still use that account for Mail, Calendar, etc.