I don't think it's sign of aggression. Well, darting into another duck, or chicken is their way of telling them to go away. They often do it in a playful way, not necessarily as an act of aggression.
I have had a drake that did that all the time to my dog, since they were raised at the same time. Puppy was one month older. Well, the drake would dart into the dog and he would run away. In case the dog didn't see him or if he was too busy digging a hole or sniffing, the duck would just shake his behind a few times and wander away. He never bit him or me hard. Nibbled, yes, darted and jumped into as well. He even seemed to have talked back. All of that seemed a bit aggressive at first, but I realized he was not trying to get above anyone. He was never mean to his drake companion or his hens. He would dart to scare crows because he thought it was cool thing to do - and both the drake and the hens started doing it as well. When the two drakes were gone, hens stopped it completely. It was extremely funny to see a flock of 10 or more crows flying away when he started chasing them. Damn, I miss him!
Sometimes ducks can be mean! They can become aggressive and try to be dominant and get their rank above yours or above chickens, other ducks,... In this case, you would most likely notice the difference. It would bite, pull out feathers, yell ...