I use the Grab feature from LinkedIn to gather contact details from emails and also manually sync some contacts between work & home. Sometimes Outlook will create a new contact rather than updating the existing one which is a hassle. Tonight I stumbled on an easier fix. Just make sure the name is the same, delete the newer contact, then Undo immediately. Instead of restoring the duplicate the new details are merged to update the old contact.
I have two contacts with Fred A’s old & new details. You could move this to another folder then back to prompt a merge changes but delete, undo does the same.
Select the new one, Delete, then immediately Undo
The changes are merged by default