@codetrekkie I don't think it is uncommon for men. It depends on the situation where you use nani-nani and the accent, tone and so on. I personally would not use nani-nani in ordinary life, but I with some times see nani-nani used in comics. I think people in real life barely use nani-nani, and that's why using nani-nani overly in real scenes could give something funny nuance to the conversation like cracking a joke, making fun of someone.
Just give you an example.
Your partner: 話があるの（We need to talk）. You: なになに？
The more serious your partner is, the more feelings being made fun he or she gets and vice versa.