> Thanks for your answer but I don't understand it, what is 
a
> query string

The things after the "?" character in an URL. For example, 
in <http://www.example.com/index.php?page=1>, the query 
string is "page=1"

> This is perfectly normal, nothing to do with my problem
> though, which seems to be related to the spider 
encountering
> several times the _same url_.

It is not the same URLs:

<http://fcpe.lattescollege.free.fr/IMG/>
<http://fcpe.lattescollege.free.fr/IMG/?D=A>
<http://fcpe.lattescollege.free.fr/IMG/?N=A>
<http://fcpe.lattescollege.free.fr/IMG/?M=D>
<http://fcpe.lattescollege.free.fr/IMG/?N=D>
<http://fcpe.lattescollege.free.fr/IMG/?D=D>
<http://fcpe.lattescollege.free.fr/IMG/?S=A>
<http://fcpe.lattescollege.free.fr/IMG/?S=D>
<http://fcpe.lattescollege.free.fr/IMG/?M=A>
..

These different URLs are generating the same content, BUT 
are different URLs that *may* generate different content. 
Neither HTTrack nor any other regular browser can guess 
that, and hence browsers will load each page, and offline 
browsers like httrack will copy them all.