Yes, they will be very close in frequency. If you have multi-path return you will have two more or more frequencies with different phases. The problem is not so

Yes, they will be very close in frequency. If you have multi-path return you will have two more or more frequencies with different phases. The problem is not so much this alone. All transmitters will have spurious emissions of some level and also your going to have higher order harmonics of the root frequency that will be outside the allocated spectrum. The higher the transmit power of a transmitter the more higher order harmonics you will see above the noise floor. At least for low power transmitters you don't have to worry about more than 2nd order harmonic. Although if you have a powerful enough transmitter like a cell tower 3rd order harmonics can be a problem. Spurious noise is going to depend a lot the design of the transmitter and amplifiers and filters. So how much noise leaks into this bad used for aircraft altimeters is gonna depend alot on the quality of the equipment cellar providers use. Noise can effect radar more easily than normal communication transmissions.