Use SR 830 lock-in amplifier to detect the second harmonic component with respect to an external reference
Maybe put a high pass filter in to filter out the first frequency component. Then it would have to lock-in to the harmonic. Don't know if this would actually work in your application.

Most lock in amplfiiers have a frequency to set to lock into, and you may be able to set it to the harmonic, and it would lock in on that.