"Welcome to the flock!"
If the fighting is excessive, separate the newcomer, but keep the pens beside each other for a few days so they can see each other and get used to each other without fighting. If you are trying to integrate an aggressive duck with a non-aggressive duck, put the aggressive duck in the non-aggressive duck's pen. Then at least the non-aggressive duck has the home field advantage!
Remember, for biosecurity reasons, it is wise to quarantine any new birds for four weeks before introducing them to your existing flock. We go into more detail about that here.
We have tips for introducing new chickens to an existing flock here. Some of the tips in that article may help you introduce aggressive ducks as well.