FileVault2 Asking for Password Twice?

Question

I recently enabled FileVault on OSX Lion. All seems to have gone well - it asks for my password before booting as expected, however it asks for it twice. Basically, it asks for my password, boots for a 30 seconds, restarts, asks for my password again - and then everything boots OK.

1. Is this (annoying) normal behavior? (From everything I've read - it isn't).
2. If not, does anyone have a remedy, or at least suggest a place to start looking as to why it might be doing this?

Hardware: MBP 13" Early 2011.