Actually, it's the opposite, there will be no duplicate due to post if and only if post urls are HTTP 301 redirected to the corresponding topic one (with pagination taken care of, as the zero duplicate is doing).
HTTP 301 redirection tells bots that the redircted url (eg the post one in our case) should never be used any more, and that the only duplicate free one, the topic url, should from now on be considered as the only one to use.
Post url are the #1 duplicate source on phpBB forum (as well as many other forum scripts), since there is many post on a topic page. If you do nothing you'll have as many duplicate for as posts in a topic.
Hence the zero and the no duplicate mods
