'How are you doing?' is more informal than 'How are you?'. It suggests that you are making an enquiry about a person's progress or health. It may not be appropriate to say 'How are you doing?' in a formal situation with a stranger, or a superior in a business context, for example. 'How are you?' can be used in any situation.

Ｈｏｗ　ａｒｅ　ｙｏｕ　ｄｏｉｎｇ？　ａｎｄ　Ｈｏｗ　ａｒｅ　ｙｏｕ？mean the same thing. The word "doing" is probably from an older way of saying "how are you?" Which is "how do you do?" It was used more for introductions I think. Anyway, we say "How are you doing?" a lot in American English. You can answer the question by saying something like this: "I am doing well." or "I am doing poorly." or simply, "Great!" or "Not well..." This probably doesn't clarify things for you and for that I apologize. Perhaps an English teacher can give a more detailed answer.

completely same meaning!

As a native speaker I use both interchangeably.

As a native English speaker, I would say that both have the same meaning to me.