lucy24 - 1:25 am on Nov 3, 2012 (gmt 0)
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).
Once you are in WordPress, the "real" underlying name of the page isn't html any longer though is it? It's .php. So you're not rewriting "blahblah" to serve content from "blahblah.html" or "blahblah.php" or extension of your choice. You're rewriting "blahblah.html" to serve content from "blahblah.php". Sure, this can be done. If you know what you are doing, you can achieve just about anything with mod_rewrite.
But among Good Rewriting Practices, making extension A serve content from extension B is probably pretty near the bottom of the list.
Now, personally-- as a user-- I don't care for extensionless URLs in any case. Whenever I see one my first reaction is to tell it to go home and put some clothes on. But that's me ;)