Lucy...correct me if I'm talking about totally the wrong subject...but I still don't see the problem.
The .htaccess script (which is common not only with wordpress ( [codex.wordpress.org...] ), but with drupal and other CMS'es), merely as apache ask...does a file exist with that file name already? If not, send it to a php script (index.php) which will match the url values to database values to see if the page exist. If it does it will be served up no problem...if not, wp displays a 404 page.
The end user or google doesn't have a clue that this happens nor cares...even if they examined the request and response headers...because this is orchestrated by apache behind the scenes.
There won't be duplicates nor stemming. If I create a wp page that says sample.html...only that is served. Not sample.htm nor sample (the latter two result in 404's assuming the .haccess is configured as I showed). On my wordpress setup sample.html/ redirects immediately to sample.html which is perfect. A lot of people configure their wordpress installs like this and don't have any problems.