FileVault2 Asking for Password Twice?
FileVault will ask for the disk encryption password to unlock the disk before booting the OS - once the disk is unlocked, it proceeds with booting as normal, at which point it will ask for your login password. Only certain users are allowed to unlock the disk. So, when you first turn it on, it'll ask you to authenticate to unlock the disk, then it'll ask you to authenticate to login. This sounds like what you're experiencing, and it is normal.
EDIT: Check the answer below, this is in error. Are you unlocking the drive with the same account you're logging in to?
This article goes into more detail. Specifically:

Upon selecting &quot;Turn On FileVault&quot;, if your Mac has multiple user accounts, you will be asked to identify the user accounts that will be allowed to unlock the encrypted drive (to start the computer or recover from sleep or hibernation).
Users not enabled for FileVault unlock will only be able to log in to that Mac after an unlock-enabled user has started or unlocked the drive. Once unlocked, the drive remains unlocked and available to all users, until the computer sleeps, hibernates, or is shut down.