In your On-Camera Flash menu, there should be an option for it to be enabled or disabled; note that even if it is disabled, it will still flash. The flash is a pre-flash that sends out the ETTL settings to the slave unit, and this occurs in the moment before the shutter is released. If your on-camera flash is enabled it will fire twice in rapid succession, once to send out the ETTL signal and again to contribute to the exposure. It happens fast enough to look like a single flash.