This is exactly/precisely the option i came here to ask about!

In the FAQ there is a note about older filetypes and dumb servers requiring
unnecessary re-downloads.
<http://www.httrack.com/html/faq.html#Q3b>

A simple switch to not grab files that already exist would be good.

Better:  allow a filter to set which files need to be updated and which don't. 
For example, "don't update files  *.wmv, *.jpg, *.mpg, *.mov