Duplicate Content

Hi i have a question about the duplicate content / link structure within wordpress

Hi i have the link structure

/%category%/%postname%.html

If i create a category (bits) a sub category (bobs) and add a post to that sub category (widgets.html) the url looks like this

example.com/bits/bobs/widgets.html

Which is what you would expect. However if you type in any of the following in to the address bar you are served with the content of example.com/bits/bobs/widgets.html even though the url is different.

example.com/bobs/widgets.html
example.com/bits/widgets.html

Each one have the server response code 200 which is not what you would expect as only one truly exists the other 2 should be 404s.

Why does wordpress do this an how can i prevent it.

I have added the canonical link/tag to the top of every page so search engines know what the url should be, but its not the best solution.

Cheers