Yes, multiple signals can exist on the same frequency at once. In a net, this could happen if two people tried to talk at the same time. This is called a "double". When this happens, the signals mix, or add together.
If the two signals are AM, the amplitudes of the radio waves add, which corresponds to the amplitudes of the audio, so you get pretty much the same effect if two people were talking at once -- you hear them both.
For FM, when the radio amplitudes add, it doesn't help the audio at all. Instead, the FM receiver will try to lock on to one signal ("the capture effect") and if one signal is stronger than the other, it will lock on. You'll still hear buzzing interference from the other signal. If the two signals are close in power, the receiver will alternately lock on to one and then the other, possibly getting nothing intelligible from either, or maybe getting short pieces of each alternately.
With FM, it is possible to capture both signals using software defined radio, and then subtract the stronger signal and recover some of the weaker signal, but this is more theoretical and not commonly done.
To the antenna, when it gets the energy from both signals; it doesn't see them as separate signals. It also gets the energy from all the other frequencies around it, some of which it absorbs and these all mix and appear as voltages and currents on the antenna feed line. This signal is then fed into your radio, which has to amplify and discriminate the desired frequencies from, and then demodulate those into the information -- possibly audio -- that you want.