Bearing in mind that Google robots scan pages by URL addresses we come to the conclusion that Google also reads tag pages. WordPress default settings allow crawlers to index these pages so all tag subpages end up in Googles index! Related to this is the problem of content cannibalization. Keyword cannibalization is a condition where two URLs of your page are competing for rank for the same keyword. Take for example a blog post on How to wrap a gift?. Lets say you want to rank high for such a phrase so you add a tag that reads how to wrap a gift. You know what will happen then.
Two new URLs will be created when the post is published /entry/howtowrapagift/ – at this address you will find an article /tag/jakpackagegift/ – at this address you will find a tag page with a link to an article about this topic. Now when a user Latest Mailing Database types the phrase how to wrap a gift into Google the search engine doesnt quite know which of these addresses to show as a search result. Its common for there to be noticeable jumps in position your site is sometimes in the top and sometimes far behind. The moment your page is high an entry appears as a search result while when you are far away the search result may be just a tag page that does not have enough content to appear in high positions.
Using a tag identical or similar to the title of the entry breaks the rule that says that one URL can have content on one topic. In my opinion if you want to use tags for navigation which is a pretty good idea from a UX perspective you should take care not to index them. It is best to do it using the robots.txt file and disable it in the SEO plugin. Tag pages should also not appear in the XML sitemap.