Support for this mod is provided in the Zero duplicate forum.
Release thread : [Simple] Zero Duplicate V0.2.2.

index.php?c=xx => catxx.html

viewforum.php?t=xx => forumxx.html

viewforum.php?t=xx&start=xx => forumxx-yy.html

viewtopic.php?t=xx => topicxx.html

viewtopic.php?t=xx&view=prev/next => topicxx.html

viewtopic.php?t=xx&start=xx => topicxx-yy.html

viewtopic.php?p=xx => topicxx.html or topicxx-yy.html if required

profile.php?mode=viewprofile&u=xx => memberxx.html

Will get rid of SIDs and mark Vars in URLs for guests :

topicxx.html?sid=SID => topicxx.html
forumxx.html?mark=fids => forumxx.html

Will check $start consistency :

Example with 25 topics per pages :
forumxx-32.html => forumxx-25.html

Will by default HTTP 301 redirect index.php :

http://www.phpbb-seo.com/en/index.php => http://www.phpbb-seo.com/en/forum.html

Will automatically follow the eventual settings in the phpBB SEO mod rewrite like :
example.com/phpBB/ => example.com/phpBB/index.html

As HTTP 301 are to be used with caution, the downloads are only available for registered users.
This mods makes HTTP redirecting very easy and you should not change all your URLs every other day.
Do not hesitate to ask for support when first installing.