"Duplicate content" 1 URL with 4 variations...

So im getting duplicate content warnings from SEO tools, its one URL but has a few variations.... Ie: one hasnt got "www." one hasnt got "/" at the end one has got".html" one hasnt got "www." ..... whats the best way to deal with this issue? Thanks...

As long as all of the pages do have the same canonical URL, it really shouldn't be a problem,
Just to make sure, do redirect all of them to a single one to make things even better

I recommend you setting up your .htaccess file up correctly with redirect rules, if this file is setup correctly for your Apache server all URL's will be automatically redirected to the correct formatted URL