You may want to install the no duplicate mod
The mod rewrite base does not rewrite post urls as topic urls and does not do it with title injection by default for several reasons.
First, post urls are in the end the only url that will be always valid to reach a message as long as it's not deleted, whatever you do like merging thread moving messages and etc ...
So keeping them keeps this property which can be useful to some. Of course, we deal with duplicate in several ways. First, the suggested robots.txt makes sure that post urls won't be indexed, so it's pretty much harmless to let user use them.
Then, the zero duplicate mod
can redirect them (if post redirections are at least set to 'guest'), and this can be performed for guest (thus bots) only or for all users.
And in then end, for those would would still like to got one step further, the no duplicate just translate post urls to the corresponding url. The link can fail to lead to the exact post if the message was moved or if pagination parameters changed, but these cases are not so frequent, so it's not really problematic in most cases.
The last concern is that the no duplicate will need an extra SQL join while building the forum indexes, to get topic titles, since it adds a proper INDEX on the topic table, it's pretty fast, but still, it can be problematic for big boards with limited resources.
So that's why it comes separately and can be deactivated in ACP once installed, even though again, it should not be a problem in most cases.
Our demo server is using it : http://phpbb3.phpbb-seo.net/
and it's included in our SEO premod