As mentioned in 3.41-beta-12 bug -
<http://forum.httrack.com/readmsg/15287/index.html>
beta-13 also redownloads unchanged files.
E.g. mirroring www.httrack.com except forums:

12:57:48	Debug: 	same level link authorized:
www.httrack.com/httrack-previous.exe

12:57:48	Debug: 	(wizard) explicit authorized (+*.exe) link: link
www.httrack.com/httrack-previous.exe at www.httrack.com/page/2/en/index.html

12:57:48	Debug: 	(wizard) shared foreign domain link: link
www.httrack.com/httrack-previous.exe at www.httrack.com/page/2/en/index.html

12:57:48	Debug: 	Changing link extension www.httrack.com/httrack-previous.exe
to .18.delayed

12:57:48	Info: 	engine: save-name: local name:
www.httrack.com/httrack-previous.18.delayed ->
web/delayed/httrack-previous.18.delayed

12:57:48	Debug: 	Final type for www.httrack.com/httrack-previous.exe:
'text/html'

12:57:49	Debug: 	File confirmed (size test):
www.httrack.com/httrack-previous.exe (3667)

12:57:49	Debug: 	Final type for www.httrack.com/httrack-previous.exe:
'application/x-msdos-program'

Size test says 3667 true size is 3755091