Google webmaster tools always complained about duplicate page titles for my s9y blog. These pages were generated by serendipity when next page is clicked at the bottom of the home page for blogs that are pagenated. It always bothered me that google didn't like that I had duplicate titles but I never really bothered to think about fixing it.
I finally got around to a fix...
{if $footer_currentPage != 1 && $entry_id != true && $staticpage_content == ''} - Page {$footer_currentPage}{/if}
Just place that code into the <title></title>
This change will number page titles whenever a next page is clicked. It will not number pages on single entries, the home page of the blog or on static pages.
The default title tag in serendipity is
<title>{$head_title|@default:$blogTitle}{if $head_subtitle} - {$head_subtitle}{/if}</title>
just change it to...
<title>{$head_title|@default:$blogTitle}{if $head_subtitle} - {$head_subtitle}{/if}{if $footer_currentPage != 1 && $entry_id != true && $staticpage_content == ''} - Page {$footer_currentPage}{/if}</title>
You can test the results by clicking on next page on the bottom of the
home page of this blog.