Imagine someone make a neder not to eat pork. Is that neder valid? If he eats pork, is he also responsible for not respecting his neder or only for doing something against Halakha?

Imagine someone make a neder not to eat pork (this is not a general Neder not to eat meat rather specifically prohibiting pork which is already forbidden). Is that neder valid? 

If he then eats pork, is he also responsible for not respecting his neder or only for transgressing Halakha of not eating pig?