Apple Contacts: Merging Duplicates
"Look for duplicates" isn't finding all the duplicate cards and when I select two cards to merge the Merge option isn't available in the Card menu but shows "Link Selected Cards" instead.
MacBook Pro
Please see this Support document regarding Linked Contacts:
Link contacts in Contacts on Mac – Apple Support (AU)
Let's say you have iCloud contacts and Gmail Contacts and Yahoo Contacts and there are duplicates between them. That is what linking will resolve. It will show as one Contact in macOS if you link them.