What would happen if Devil Fruit that Chopper ate were eaten by a human being? Wiki about Hito Hito no Mi:


  The Hito Hito no Mi is a Zoan-type Devil Fruit that allows its user to
  transform into a human hybrid and a full human at will. It was eaten
  by Tony Tony Chopper.
  
  By consuming the fruit, Chopper has been given human intelligence,
  allowing him to understand and communicate in human language. The
  fruit has also given Chopper the ability to perceive concepts that a
  reindeer would not comprehend, such as medicine. It also allows him to
  transform into human/reindeer hybrid forms.


So how about if the fruit were eaten by a human being? Would the effects of Hito Hito no Mi kick in?

And it looks like Chopper ate that fruit by accident?
Most likely nothing would happen, that person would lose the ability to swim but would not gain anything by the fruit itself. This was also answered by Oda in one of the sbs questions.


  
    D: WHAT HAPPENS IF A PERSON EATS A HITO HITO NO MI?[6]
  
  
  O: I suppose I should say they "BECOME HUMAN". Uh, I guess you could
  say that means like becoming an adult, or finding your true spirit or
  something. So basically, people will live more human-like. Well. I
  guess you could say that. Or not. Basically, not. Goodbye. (runs
  away).


http://onepiece.wikia.com/wiki/SBS_Volume_20

EDIT: maybe, the only thing a human would gain is the ability to ascend("to get to a higher state of consciousness") so one would be happy with himself, so to say. However this is just a speculation, and my take on it if something like that would happen and Oda would come up with a "power up" for the user