Without more info, I can only guess that they’re not actually exactly simultaneous.
If that’s the case, the original waveform and its copy combine into a new waveform with a new pattern of peaks and troughs, interpreted by our ears as different timbres (textures) and sometimes even pitches (due to new frequencies being formed), though the overall fundamental frequencies will remain about the same. This is because you’re adding the value of the two waveforms’ deviation from the center at every point in time, and if they’re not completely even in time, sometimes one copy of the wave will be going down when the other is going up, meaning you might actually lose total deviation from zero at a given point in time when compared to the single original wave at the same moment.
Essentially, if the start times are close enough, the combination of two copies of a sound will sound like one sound that’s similar to the original, but with different timbres. However, if they’re far enough apart in time, your brain can separate them into two different information streams and it sounds more like an echo. Only if they are exactly simultaneous will they combine perfectly and act like a louder version of the original sound wave.