A standard WordPress site has the same content displayed in numerous places, such as Archives, Categories, Feeds and so forth. Blog Rambler says that this can damage your Google ranking since Google penalizes duplicate content:

When a crawler visits the site it must decide which of these pages is most relevant, and the majority of time it does not pick the one you would. I have tested this on several sites and found that the Category pages usually get listed ahead or in place of the actual post.

Read the solution at WordPress Duplicate Content on Blog Rambler>>