Volume Encryption plus FileVault -- redundant?
Don't think it's double-encrypted. I suspect you created a 'Disk Password' when encrypting in Disk Utility and then you added users to unlock the disk when enabling FileVault. Out of curiosity, what do the following show?

diskutil cs list
sudo fdesetup list -e


Answers from original poster:

diskutil cs list shows:

+-- Logical Volume Group B48571B2-985A-48B3-91F0-0C9AA9FFBE0A
|   =========================================================
|   Name:         Macintosh HD
|   Status:       Online
|   Size:         511250432000 B (511.3 GB)
|   Free Space:   36864 B (36.9 KB)
|   |
|   +-&lt; Physical Volume C60946EB-D0F1-4A80-BAAF-6D7F0B716C2A
|   |   ----------------------------------------------------
|   |   Index:    0
|   |   Disk:     disk0s2
|   |   Status:   Online
|   |   Size:     511250432000 B (511.3 GB)
|   |
|   +-&gt; Logical Volume Family 3C90D9A1-05C4-4D94-B20F-05F469418176
|       ----------------------------------------------------------
|       Encryption Type:         AES-XTS
|       Encryption Status:       Unlocked
|       Conversion Status:       Complete
|       High Level Queries:      Fully Secure
|       |                        Passphrase Required
|       |                        Accepts New Users
|       |                        Has Visible Users
|       |                        Has Volume Key
|       |
|       +-&gt; Logical Volume 38FE20AC-17E6-4C37-B465-FB270BE91718
|           ---------------------------------------------------
|           Disk:                  disk2
|           Status:                Online
|           Size (Total):          510883004416 B (510.9 GB)
|           Conversion Progress:   Complete
|           Revertible:            Yes (unlock and decryption required)
|           LV Name:               Macintosh HD
|           Volume Name:           Macintosh HD
|           Content Hint:          Apple_HFS


sudo fdesetup list -e shows:

ESCROW  UUID                                                                     TYPE USER
        799FBB35-D5A3-443E-B397-E934AECC6D51                                  OS User myusername
        1B4714C8-EB0E-42D4-A2D4-F45E19CA6890                              iCloud User