What creates intermodulation interference between two repeaters in close proximity?
Why Intermodulation needs two signals and a nonlinear device to mix them. When repeaters share a site, RF from one transmitter's antenna gets back into the other transmitter's final amplifier, and that final acts as a mixer, producing sum and difference products that are radiated as spurious signals. The standard cure is a ferrite isolator or circulator at each transmitter output, which passes power outward but attenuates signals coming back in.
Watch out The harmonic-relationship choices describe simple harmonic radiation, which is a separate problem; intermod products occur between any two frequencies and do not require them to be multiples of one another.
Intermod is mixed in the final. Fix it with an isolator, not a harmonic filter.