See: http://googlewebmastercentral.blogspot.de/2009/02/specify-your-canonical.html

The following 4 URLs all point to the same page:
http://www.phpbb.de/community/viewtopic.php?f=93&t=186668
http://www.phpbb.de/community/viewtopic.php?p=1082831#p1082831
http://www.phpbb.de/community/viewtopic.php?f=93&t=186668&start=0
http://www.phpbb.de/community/viewtopic.php?f=93&t=186668&sid=abcdefghi...

By adding <link rel="canonical" href="http://www.phpbb.de/community/viewtopic.php?t=186668" /> to the head section search engines are being told that it is the same URL. This avoids duplicate content reports in webmaster tools.

We should do that for the following files for now:

• viewtopic: start=0, p=, highlight=, mark=
• viewforum: start=0, mark=
• memberlist&mode=viewprofile: un=username