Hi John,

If by 'first harmonic' you actually mean the second harmonic of your LO
(the 'first harmonic' usually refers to the fundamental frequency), the
answer depends a lot on how well your input is filtered, and on what
you have after the mixers.

This sounds like a 0-30Mhz shortwave receiver. If your input does not
have a good low pass filter for example, you may find strong FM signals
getting into the first IF passband as a result of the 2nd LO harmonic.
Note that your mixer inherently responds at odd harmonics of the LO
(3rd,5th...) and filtering odd harmonics out at the LO port does
little, so a good low pass filter is necessary anyway.
The second mixer is probably not as critical. Others will probably add
some more information.

Regards,
Glenn AC7ZN