To stop spam, Google and many other search engines impose a Duplicate content penalty.
This might pull down search engine rankings. Blogger archive pages can pose a little problem . You can see on this image below that both archive page and post pages have the same content. You need to block it.
This is very simple actually. Just go to Blogger Dashboard –> Layout –> Edit HTML
Add this just after <head> tag.
<b:if cond='data:blog.pageType == "archive"'><meta name="robots" content="noindex, follow" /></b:if>
This code instructs crawlers not to index archive pages.
I recently posted an article on shoutmeloud.com about SEO tricks using Google Webmasters tool. You can check it out.