What is considered duplicate content?

I run a free classifieds website http://www.jihoy.com, and noticed that one of my users are posting over 70+ ads, mostly with different titles (bypassing my duplicate checker) but similar contents.

http://www.jihoy.com/profile/kondas005

Now, if they have slightly different titles, and slightly different descriptions, are those considered duplicate contents by Google? What's the % duplicate in content before it is considered as duplicate content?

 

it's tough to call but copyscape gives you a clear indication if content is duplicate or not.