I had the same problem some time ago. It created thousands of non-existent pages across one of my sites.
This is what I used:
RewriteRule ^([^.]+\.+html)/$ http://www.example.com/$1 [R=301,L]
I guess lucy24's way of doing it is more comprehensive, so stick with it.
In Google WMT, just select all of the pages that appear, and mark them as fixed. Don't get surprised if many of new ones start appearing every day.