In the freepbx interface I see 4 types of voicemail:
I have 2 wav files, busy.wav and unavail.wav. When the file is played another message is played which says something like “you can leave a message now” but this message is already in the custom wav files so the caller is prompt twice. Can I turn that off?
What files need to be used for “no message” and “instruction only”? And what are the differences?
“No message” just plays a beep.
It’s useful for the occasions when you want to play a different message before the beep. Instead of sending the call to one of the standard voicemail destinations, you create your own by (for example) sending the call to an “Announcement” that has (as its following step) Voicemail with No Message.
“Instruction only” just plays the message about leaving a message, and then dumps the call (IIRC). I’ve never found a real use for the Instruction Only option.
Now I still not know what to do 
Then ask a question that we can answer.
You asked what file is used for “no message”. It’s like asking “how tall is green?”
I know English isn’t your first language, but unless you can tell us what you want to know, we just have to guess.
These are my questions:
@Cricchetto just asked this question about 5 minutes ago. The answer to you is the same as the answer to him.
I have two different messages and I don’t see how the correct one is played just by setting the voicemail destination to no message.
If you are using the “no message” type, thre is no message. All that happens is the caller gets a “beep”. That’s why it’s called “no message”. You’re expected to play a message (announcement) of your own before you send them to the voicemail destination.
That is what I doing, I have multiple voicemail messages, but as I said, after the message is played, another standard freepbx message is played which basically repeats what is already in my own message.
Your announcement is sending the call to the wrong voicemail destination. If you are getting a message, you are not sending the announcement to the right voicemail box.
Another possibility is that you are not using the “optional destinations” correctly. You should send these to an announcement (and not a voicemail option) then send the announcement to the “no message” mailbox for that user.