A Fake Friend
If you get a Facebook 'Friend Request' from somebody that's already your friend, it's a SCAM!
Before doing anything with the email, check Facebook and confirm you are already friends. If so, the email is just from a scammer trying to get to your profile (because your friends can see your stuff).
How Do I Fix it?
Click on the name in the email (NOT 'confirm request'!) and it will show you the profile. Or you can access the Profile Request from Facebook ("Friend Requests" is a 'people' icon near the top).
Usually it's just an empty profile that has 'borrowed' the name and profile picture of your friend.
Click on the 'dots' in the profile, and choose 'Report' and then 'Report this Profile'. You can copy your real friend in the process.
I have found that Facebook is quick in policing these profiles.
If you accidentally do confirm a profile, you can still use the same process to report it and then unfriend them.