Why do I have two Guest Users?
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.