There was a big discussion around this a few weeks back...

http://www.flyertalk.com/forum/Forum5/HTML/006623.html

The discussion boiled down to 'it seems to be illegal but plenty of people do it and don't get caught'.

While obvious BtB circumvents airline pricing rules (and illegal), nesting tickets (ie buying two rt tickets for the same city pair and then using them 1-2-2-1) is not. But, as you will see in the thread, plenty of people do it and get away with it...