Its not that it actually overloads the IF; if the receiver is a single conversion superhetrodyne, then two different frequencies can beat with the local oscillator to create the IF (intermediate frequency). The most common IF is 10.7 MHz, so one frequency 10.7 above the local oscillator frequency and another frequency 10.7 MHz below the local oscillator frequency will both produce an output centered at 10.7 MHz. The tuning of the first stage is supposed to suppress the unwanted frequency.
(Remembered this in the shower) Also, I have noted that in cheap FM receivers with digital synthesizers, for reasons that are not entirely clear, a clean oscillator at a single frequency frequency will show up on several spots on the dial. This might be the result of the receiver being all on one chip and the reference for the synthesizer and its harmonics get into the wrong places on the chip. Its best to not use cheap digital tuning receivers that have digital tuning while turning in your transmitter.