http://forum.httrack.com/readmsg/4053/4047/index.html

Suggests that it should skip the file that is already downloaded.

<https://forum.httrack.com/readmsg/26644/26643/index.html>

Says "nope"

<https://forum.httrack.com/readmsg/32489/32488/index.html>

Says, "HTTrack will overwrite anything that gets in its way when it is
downloading."