Enquiries about Key (The song in Duty)

[mellody]

Since i haven't know much of younger Ayu..
I want to ask this..

The song Key ~eternal tie ver.~
Is that the only "Key"?
I mean, does she have Key (Original version), or something like that?

Because it's kinda strange, the song doesn't have any original version,
Anyway, maybe the title have any connection with the lyrics (of course)..

Anyone can explain?