asterisk users - May 2005 - Registering phones with the same/invalid extension number

I have asterisk up and running now, and installed XLITE on 2 PC's.  Both
machines (mistakenly) registered as the same user / extension.

Strangely, asterisks allows this and both phones can make calls!  But,
only the first one to register can receive calls at the extensions.

1.  Is this normal behavior?  (Why allow 2 phones on same extension)
2.  Why is asterisk not showing the second phone when I type SIP SHOW
PEERS
3.  Is there some way to make asterisk NOT allow phones to connect on an
extension already in use?
4.  Is there a way to make asterisk NOT allow phones to connect that do
not have a valid extension in SIP.CONF?

Thanks,
Mike