# perl rename
rename 's/(?:\.html){2,}$/.html/' *.html

##</code><code>##

# unix rename
rename .html.html .html *.html

##</code><code>##

# perl oneliner
perl -e '-f && ($a = $_) =~ s/(?:\.html){2,}$/.html/ && -e $new ? warn $_ : rename $_, $new for @ARGV' *.html