It's just that many times, people post more backlinks without the index.php to link the forum index. On most phpBB forum, you end up with both -www.example.com/phpBB/ and -www.example.com/phpBB/index.php ranked and working.
It's duplicate content on a very important page, so we want to either redirect one or the other but not keeping both.
And as we can easily manage to get rid of index.php in phpBB links with this mod (doing it by default) and redirect index.php (as here :
http://www.phpbb-seo.com/boards/index.php ), and we cannot easily change all external backlinks, we should go for the most handy and used standard.
If as many other you ended up with more external backlinks without index.php, you should get rid of it inside the forum.
Usually, the page getting the most PR between the two possible duplicates should be the target, but if you've no reasons against it, getting rid of index.php is the best as people will again most likely not include it when back-linking.
This being said, the zero duplicates makes both choices almost identical SEO wise, backlinks using the two kind of url will be properly redirected (http 301)
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