I know how to create a copy of a local website.
"httrack -N99 <file:///home/user/web/index.html>

But it seems that when I update the copy ("httrack --update"), all files are
recopied instead of only the files that have been changed since last time
httrack is used.

I also get a "damaged cache" error, only when copying local websites (using
file://).
Warning:        Cache: damaged cache, trying to repair
Warning:        Cache: 0 bytes successfully recovered in 0 entries
Warning:        Cache: error trying to open the cache