We do that so you can choose to use the shorter URL if you’d like.
I suppose you could create a redirect for every individual URL (there is no facility for *.html => *(without html), or vice versa), but that seems pretty painful, and I can’t see the point of doing it.
The real solution is as I mentioned: not to link to the one you don’t use; nobody is going to “guess” it, least of all search engines, whose opinions are the only ones that I can see that might matter…
If you have another concern besides the potential SEO hit for duplicated content, I’ll be happy to talk through it with you if you would elaborate.