You should not put the same ad unit more than once on the same page of your website.
Why?
* The ad units that have the same ID will be stuck together when displayed.
* Our partners detect the number of times the same ad is displayed on the page. The revenue will simply be divided by the number of formats.
However, there is an exception for the “Rectangle” and “Megabanner” ad units. Those ones are available twice by using the “Bottom Medium Rectangle” and “Megabanner bottom” tags.