Redux has been agreed upon across the board as the Flux implementation. Flux is a great idea, but it left a lot of questions that individual developers attempted to answer with their various libraries / implementations. In its current iteration, Redux implements what many consider to be the most important parts of Flux (uni-directional data flow) without making use of the unnecessary parts of Flux (the dispatcher for instance).
If you are interested in hearing a more eloquent description of the differences from the creators of Redux and Flummox, listen to this Javascript Jabber episode.