There are a couple of reasons not to do this.
- Pages are crawled and indexed. Now, two identical pages cause difficulty. Google, in attempting to offer relevant material, will often go with the original, filtering out a secondary copy.
- Web traffic may be diluted so you actually earn less for two pages of identical content. Far better to spin the second copy into a different article.
- Then there's the dreaded Duplicate Content Penalty. If you haven't heard about it, you need to be aware that content producers have, in some cases, been banned from the Google AdSense program, resulting in loss of accrued and future AdSense revenues.
I've prepared Google's Duplicate Content Rule: Information That Sheds Light on Google's Policies. I hope this article helps to clarify issues that concern webmasters, publishers, and others who produce online content.