Why do you believe is better to display - excerpts or the full blog post? I'm inclined to display the full posts for the reason that it's better for the readers. I beleive that it keeps them on the page longer as it's easier to scroll down the page.
The issue is that the duplicate content will slow down the crawl rate. As result less of your pages may get indexed.
Two ways to show excerpts on your wordpress blog are:
(1) Replace the_content() function on your pages with the_excerpt() which displays the first 55 characters of the post.
(2) Use the <!--more--> tag which will automatically create an excerpt at the point that you place the tag.