If you have a setup where your luks-encrypted device is a LVM partition which includes all mount points listed in /etc/fstab, you may be asked to enter your passphrase twice during boot: once by grub and again by systemd.
To have the system only prompt once, first get the correct device path and UUID with the following command:
$ eval $(lsblk -npfl | awk '$2 == "crypto_LUKS" {print "DEVPATH=" $1 " DEVUUID=" $3}')
$ sudo cryptsetup luksDump $DEVPATH
NOTE: If no output is returned, it's best to stop here and double-check your setup before continuing.