Is there a known vulnerability when using identical key and salt with PBKDF2?
Using the password itself (or anything similar predictable) instead of an independent random value as the salt denies the whole benefit of salt:


Same passwords (passphrases) give now the same key, instead a different one. So, if two users happen to choose the same favorite image as their password, they get the same key, and thus an attacker can use this fact to easier get the key.
If an attacker bruteforces your "password" database, he can attack multiple passwords in parallel, since they don't use different (independent) salts.


If you have a way to generate good quality random numbers (which should be the case on every normal personal computer), generate a random salt and put it at the start of your encrypted file (like a normal initialization vector).