This is a naturally occurring phenomena referred to as Multipath.  It is typically seen in shallow water.  There is nothing wrong with the sonar and there is nothing you can do to correct this.  

Multipath: Sonar signals arriving at the transducer from a single source but along different paths. In the sonar sciences, multipath echoes can be problematic. Multipath returns in imaging sonars typically occur in shallow water or around complex structures such as platforms or near piers and pilings. A classic multipath environment for side imaging sonar is in shallow water with a flat/calm water surface. Acoustic echoes will return to the transducer along three different paths (1. transducer - target - transducer, 2. transducer - target - water surface - transducer, 3. transducer – water surface - target - water surface - transducer). If these three return paths take three different times, the result will be three images of the same target in the data.

Below is an image that demonstrates multipath.

SI_Multipath