Apologies.
I didn't spend a lot of time, but I did a site search, found a tagged page, looked at it - you have a breadcrumb navigation at the top of that page for tagged pages.
I also looked at the bottom of a single post page, found a link to archives, clicked on it, received a fatal error for archive pages, which seem to be the tagged pages?
SO - I'm not quite sure what the set up is, and where the tags come into it all, but probably you've got what the search engines consider duplicate content, and they're not too bright at the moment. You can specify your canonical url - information here: http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html
Simplest way, however, is to put meta noindex, nofollow at the top of all archive and tagged pages. Since archives use archive.php and tags usually use the generic tag.php that shouldn't be difficult. You could then put rel=nofollow on the archive/tag link that I found at the bottom of the page and on the top breadcrumb navigation when tags are involved.