I know there are some threads on this topic but my own problem is differ. On Google webmaster tool I have a notice of duplicate title tag, I don't expect this to happen because at the beginning of setting of the site I use WordPress SEO plugin by Joost de Valk which I expect to deal with such thing for me. After some weeks I still get duplicate title tag warning. Could this mean the plugin is not working? Please somebody should tell me what may be happening and the best solution. thank in Advance
Your Title Tag must be different of all webpages and unique too. If you changed then wait for 3-4 days when Google will crawl your website then it remove duplicate title tag warning.
As regards Joost - It won't deal with the title tag as you choose this yourself and may actively want to have the same (or similar) title tags. For example. If you are running a project and via a website and need "version control", then using the same tag might be needed. For instance -one of my sites deals with specific mechanical issues and service rotas for fork lift trucks. The same tag is appended to a new page at every service. Ok, this may not be a typical example, but as the blog owner you have absolute control over the titles you choose to use, the simple answer is don't use the same ones twice" If you have Joost's SEO plugin installed, go to the post or page "Edit" and then scroll down. The SEO title tags and main meta data are all laid out for you at the bottom of each page and post along with a mock up of how Google might present this data once you have made the changes. Paul Rone-Clarke
You should never rely on a plug in to do anything for you. Their are many good plug ins and many poor ones you never know even the good ones can get bugs and die. you might have a bad theme that makes 2 title tags.
Why don't you just rewrite it in your other pages... In most cases there is an option to use the ARTICLE title for the TITLE for the page!
It takes a while for duplicate title and description to get removed from webmasters. Check the last updated on the HTML improvements page. If the date is atleast a week after the changes, I would guess the plug in is not working
Each of your pages should ideally have a unique title tag, which helps Google know, how the page is distinct from the others on your site using a single title tag across all of your site’s pages or a large group of pages. Titles can be both short and informative. If the title is too long, Google will show only a portion of it in the search result.