[messaging] Prekey reuse

[N A]

Dear mailing list,

I read in [1] that TextSecure uses prekeys only once:

    "Since the server never hands out the same prekey twice
    (and the client would never accept the same prekey twice),
    we are able to provide forward secrecy in a fully
    asynchronous environment." [1]

There is however a last-resort key which is potentially handed out
multiple times. Could someone please explain to me how forward secrecy is
in danger when prekeys are reused? And why would this not be a problem
with the last-resort prekey? Are there any other problems with prekey
reuse?

Many thanks!

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[1] https://whispersystems.org/blog/asynchronous-security/