It is not important who he is. = Who he is is not important. Is that right?? That structure 'is' is said twice seem strange to me.
Yes, this is correct.
Regarding the double is:
- the first is applies to he
- he second is applies to who he is or his identity
You could replace who he is in this way if it helps you parse the sentence:
His identity is not important
Grammatically, who he is is the subject of the sentence, and the second is is the verb of the sentence.