Order of operations on clean High Sierra install: APFS Encryption or Filevault? Both?
Here is what I found useful for me. To avoid a double login situation, or where you see both the User login and Disk Password on the login screen after boot, do NOT select APFS Encrypted in Disk Utility when formatting the hard drive. Select the regular APFS file format, and THEN apply Filevault once you complete setup.

This applies to when you are installing from a bootable usb stick, as it was in my case. 

I hope this is helpful for you!