Finding "identical" code snippets is relatively easy, there are existing tool that already do this (see other answers).
Sometimes it's a good thing, sometimes it's not; it can bog down development time if done at a too fine "level"; i.e. trying to refactor so much code, you loose your goal (and probably bust your milestones and schedules).
What is harder is to find multiple function/method that do the same thing but with different (but similar) inputs and/or algorithm without proper documentation.
If you have to two or different methods to do the same thing and the programmer try to fix one instance but forget (or does not know they exist) to fix the other ones, you will increase the risk to your software.
