open the people app, go to that particular contact, see below it. there are generally 4 options. one is a pin, one is chain, one is pencil, one is 3 dots.
click the chain symbol. that shall help you link/ unlink.
also, if you want to hide unwanted duplicate contacts that don't have phone numbers in them,
the follow these steps:
people app> settings (three dots at bottom right)> scroll a bit down to "filter contact list"> then turn on the "hide contacts without phone numbers" option. also, you can add/ remove accounts from there.
hope I was able to help.
also, please feel free to join the windows central community. if you do so, we might just be able to help you better.