I've heard three somewhat unusual responses to this question. You can take them seriously, or not.
My late uncle, who loved to be humorously sarcastic and throw people a bit of surprise would often respond, "You'd better watch out, because I might tell you."
My grandmother, a classy woman, would actually answer a bit about how she was feeling that day - "I have a headache and arthritis and constipation and .... " After listing her woes, it always ended with, "I'm healthy!"
An interesting short polite answer that I've been hearing lately is, "Thank G-d". I thought this was an answer that was unique to the Orthodox Jewish community. However, recently, I have been hearing this from a number of non-Jews, as well. I don't know if they learned this from the other community or this is part of their religious belief, or what. But, I think it's a nice response.
Why use a rather confusing and somwhat impolite answer? I would feel insulted if someone responded to my "How do you do" with another "how do you do". If you dont mean to ask people how they are doing, then hello, goodmorning, etc should suffice.
The use of that must have been born out of pride, where some think its disrespectful to be greeted with such inquiry, so they respond with same.