Why do I have two Guest Users?

Solution 1:

The one guest account with a proper picture is an OS account - you can remove it from the FileVault startup screen with this command.

Enter the following command in the Terminal:

sudo fdesetup remove -user Guest

That guest account is still around, but it’s not the one that comes from Find My Mac and FileVault - that’s the one with the shady silhouette - you can disable that by turning off Find My Mac or going to the user accounts preference pane in system preferences and electing the guest account configuration you prefer.

Solution 2:

This is a bug. I have the same issue with High Sierra and the icon is corrupted too. MacOS has two types of a Guest user:

• An actual (but restricted) user account.
  It can access files and apps, so it can’t be used on an encrypted disk.
• A special mode that a computer reboots into.
  It can only use Safari.

For some reason Guest user account shows up on the login screen even though it can’t log in due to encryption.

I’ve just deleted the Guest account and it no longer shows up:

• Open Terminal app

• Paste this text:

  sudo dscl . delete /Users/Guest

• Type Admin password and press Enter