Multiple bottom returns are created from the sound energy bouncing back and forth between the bottom and surface of water and normally only occurs with harder bottoms. In 2D, the stronger portion of the beam is directed downward. In Side Imaging, the stronger portion of the beam is directed outward and not likely to produce multiple returns.
As a note, if you look at the left side water column in SI you can see what happens with sound reflections from the transducer probably being mounted on the right side of a jack plate and sound bouncing back from the left side of the jack plate.