I have 2 other sites that use phpbb3 and I have been looking at optimizing each site as phpbb3 does seem to have some issues with content with duplicate urls. There are several other mods available.

Two seem well-known:

They both seem good and the team from phpbb SEO makes a compelling case for their mod. I generally don’t mind paying for a mod if it makes my life easier however my big issue with these mods is keeping up with changes to phpbb. If history is any indication, you may need to make adjustments along the way.

There may be an easier way from Tech Tibits (check out the site for details):

An alternative, albeit easy way to prevent duplicate indexing is to add these lines to your robots.txt :

User-agent: *
Disallow: /viewtopic.php?p=
Disallow: /viewtopic.php?=&p=
Disallow: /viewtopic.php?t=
Disallow: /viewtopic.php?start=
Disallow: /*&view=previous
Disallow: /*&view=next
Disallow: /*&sid=
Disallow: /*&p=
Disallow: /*&sd=a

Very clever. I’ll try it out on one of my forums and see how it works out.