Duplicate paths declare listeners more than once

[jeremyharris]

Hey,

I was running into a problem where my listeners were brought in more than once in plugins (namely the api_generator plugin).

Below is a fix and test case: http://github.com/jeremyharris/referee/commit/77672e5e13c47cbd23ddc8004a8f530c01db3c47

I'm pretty new at submitting patches/corrections so let me know if I need to do a pull request or whatever.