I hear about the double bottoms showing up a lot. However, it is most common when you are using 200 kHz freq. If your CHIRP is on, and your freq reads HIGH, switch it to MEDIUM. MEDIUM or MEDIUM/HIGH should be the correct setting.
With a Helix, the unit is broadcasting 200 kHz (high CHIRP) whether you choose to view it or not. It is used for the Depth readout.
83 kHz (med CHIRP) is turned on when you choose to view 83 kHz or 83/200 kHz.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1