Well with a one year old, you don't just turn them out in a herd unsupervised. Only turn out together after the appropriate amount of over the fence time of a week or so. Yearlings are not usually strong/smart enough to fend of a serious attack. One classic way it is done it to introduce the horse one by one with the others. Lock all horses in stalls and then take the filly to the lot. Then take just one horse (start with the lead horse) out to her. If you have an unfamiliar lot to the lead horse, use that one. There are fewer issues over territory. The lead and your new horse will likely work out their issues pretty immediately as lead horses are smart and establish themselves in the flutter of an eyelash. Once the lead horse and she have an established herd order, you can put the lead horse away and try the next horse in line. Where there will be serious amounts of fighting is when she is with another horse of similar rank to her, probably near the bottom of the herd due to her young age. Let this one horse she has issues with and her duke is out but as safely as you can. Plan on only brief times together and food helps to distract them from picking at each other. Be sure there are plenty of piles of hay or nice grass so there is not a fight over the food access. Be sure she doesn't get cornered and get the snot mule kicked out of her. If you feel she will get beat up, you are not ready to introduce. More fence time.
When I do this, I'm in there with a lunge whip in my hand ready to break up any serious fighting like a lead horse would. If you trust your lead horse to be on her side, and not join in the gang up on the cute young one action, you might consider having the lead horse in there to keep the "problem horse" in line. Many lead horses will if they take the new one under their wing after days or weeks of turnout time together. You might not feel comfortable though in this role as lead mare and have good enough horse sense to place yourself in a position of authority and not a dangerous position. So use your head above all else and stay safe.
Horses need to have these interactions before there can be peace. But, most people use the fence or housing them in nighboring stalls to keep the injuries to a minimum. Young horses can get seriously injured or killed while being introduced without the protection of their dams. Your horse is too old to be standing with her dam but remember in nature, she would have her dam and her aunties to protect her at this stage. Right now she has just you and you aren't in the paddock 24/7. Do not leave her in there unsupervised until you have peace and quiet for at least an hour at a time. She could get really hurt.