How should I say "I feel very good/I'm very fine" in response to 你好吗? I've read that when asked the simple question 你好吗? ("How are you?"), answering 我很好 is the standard form, while 我好 is simply not used.

很 means "very", but 我很好 is simply translated as "I'm fine." and not "I'm very fine".

So if I want to say "I feel very good/I'm very fine", what should I say? Should I duplicate 很 or use another expression?
There is "我非常好" I'm extremely well, or I'm extremely good.

There is also "我非常开心" I'm extremely happy.

Maybe if you want to aim for something a little more subtle you could try "我很开心" I'm very happy.

If you are mentioning that you are this "good" you will likely be asked for an explanation!