Someone getting double unders on you while mounted is a gift. I'd personally move to S-Mount and get a nearside overwrap armbar, but you could go straight for the armbar, move to Technical Mount, Ezequiel choke them, Cross choke them. If some reason you really wanted their double unders broken you could put your forearm under their nose and push yourself up with two hands, but seriously this would be a godsend and no decent grappler would ever try to do this to you.