This kind of thing can happen. One thing that can help immensely is making sure there is enough elbow room in the brooder. At first, about a square foot per duck. Then as they grow, the brooder needs to grow, too. Too small a space leads to aggression, feather eating, anxiety, failure to thrive, or other less than ideal behavior.
Try a larger brooder, and set a divider fence using plastic poultry fence or similar material, so that the ducklings can see each other and get accustomed to each other. The littles will catch up in a matter of weeks, and with supervised time together, they should bond into a little flock. Once the babies are old enough and have had some grit, let them enjoy pea treats at the same time, then together, and that will likely ease the transition into one group.