Most search engines especially Google Doesn’t tolerate duplicate content. If you have duplicate content on your blog it will have negative impact and search engines will not show certain duplicate pages in the search result and it will be in Google supplemental results which is a secondary database containing pages of less importance  .
It is a common understanding that the supplemental index is Google’s way of filtering out duplicate content.
If your blog has TITLE elements, META descriptions tags, and navigational text that are identical, that can lead to duplicate or near-duplicate content.  If your page is included in Google supplemental results then it will have a direct impact on your blogs page rank. Links to multiple versions of the same page reduces PageRank across multiple URL.

How to avoid duplicate content on WordPress Blog

In wordpress, archives and categories are mainly responsible for duplicate contents. If you add one post to more than one category that will result in duplicate content. Archives also results in duplicate contents when search engine crawl your blog.

Make sure that you don’t use different categories for same post. Also there are some wordpress plugins that can help you to avoid this problem.

SEO No Duplicate

This plugin helps you manage your search engine duplicate content, by setting your post page’s canonical to the permalink.
Get SEO No Duplicate

Duplicate Content Cure Plugin for WordPress

Duplicate content cure is a very simple, yet effective SEO plugin that prevents search engines from indexing wordpress pages that contain duplicate content, like archives and category pages.
Get Duplicate Content Cure Plugin