Similar URLs for same page

Those will absolutely be considered two different pages. Just one character is all it takes to make two URLs different. Canonical URLs are commonly used for different query strings that product the same content but file extension is no different. Definitely use canonical URLs for pages like that.

If you have the proper canonical page link on the page, then search engines shouldn't treat the pages different.

However, from a SEO perspective, there's no definitive answer one way or another (Google "seo ranking file extension" to see what I mean)

What you should be concerned about, however, is that cool URIs don't change and that most people have an easier time with remembering pages without extensions than they do ones with extensions (do you really want people trying to remember if it's .asp/.aspx or .htm/.html based on your platform?).

Additionally, there's a point to be made about the extensions themselves, that they tie the link to the implementation of what's serving up the link (and that this is an implementation detail which is not specific to the content of the page itself).