From <http://www.httrack.com/html/fcguide.html>: C  create/use a cache for
updates and retries (C0 no cache,C1 cache is prioritary,* C2 test update
before)

So, in command-line mode, using -C1 instead of the default -C2 should ask
Httrack to ignore any file it already has, if I understand correctly. My
mirror is not fully finished, though - not sure if a fully finished, then
updated mirror will behave the same way.