This problem results from the use of 'itemid'. Itemid is a piece of data used by Joomla to decide which module or template should be used on a give page.

The use of Itemid by Joomla will lead many times to the so-called 'Duplicate content' issue, which is rather a duplicate URL issue : the same content can be accessed from various location within your web site, and therefore various URL. This is considered a bad thing by search engines, and will likely results in reduced ranking performance. Please note this happens regardless of whether you are using a SEF component, Joomla SEF URL or even Joomla original non-SEF URL.

sh404SEF® tries to fix this issue as much as it can, and is usually quite successfull at that. Sometimes though, this can lead to side-effects such as modules and/or templates being displayed on wrong pages.

Detailed instructions on how to fix this problem can be found in the relevant section of the User Manual.