I just happened to start up a simple hello world mocha test project (to test something else :) ), and I noticed that versions >= 2.4 now double-render the failure in my example. Reverting to 2.3.3 makes the behavior go away. I will try to get to the bottom of it, but I see several releases in the last few days so I thought maybe you'd want to know.

https://github.com/ahamid/mocha-phantomjs-blanket/tree/mocha-failure-double-render