I'm wondering about the best way to train my Bayes filter (per-user

I have a Junk folder, and it contains roughly three categories of mail
(to my mind, at least):
A. Mail SpamAssassin marked spam and auto-learned as spam.
B. Mail SpamAssassin marked spam, but did not autolearn.
C. Mail SpamAssassin did not mark spam, which I moved in there.

So my questions:
1. Would it be bad for me to just run sa-learn on the entire Junk
folder; or should I just let auto-learn do it's thing and sa-learn the
false negatives?

2. Likewise, my Inbox contains just ham; could I run sa-learn on that
entire mailbox periodically?

3. Lastly, will it be detrimental (in terms of future accuracy) to sa-
learn the same mail more than once, or will SpamAssassin remember it?
(I seem to remember reading the latter, but I wasn't sure). If it
does, how long/many previous mails does it remember?