I'm assuming you are using the 5e OGL character sheet and that you are using the setting "Auto Roll Damage and Crit". If so, the "advantage" damage dice are actually the critical damage dice. Because Roll20 cannot know if you have rolled a critical hit or not, it automatically rolls the critical hit damage dice. If a critical hit occurred, these dice are displayed in the roll template. If a critical hit did not occur, they are hidden in the roll template. In order to prevent the critical hit dice from being displayed, you can either not use 3D dice, or turn off the Auto Roll Damage and Crit setting.
