Source code cloning usually goes unnoticed until things start to go bad for the company. Managers tend to look into static code analysis and then realize that they've missed the train when cloning was still manageable and could be contained.
I like this paper since it gives a certain perspective of the code clone measurement - tracking over time. The method is not trivial to apply, but there is a tool (Java only, though) which can support automated tracking of clones.
I will set it as a project for my students to get in touch with the authors and develop a tool for other languages.