Fighting is how goats determine their place/status in the herd. When bringing new goats into a herd there may be some fighting. The best thing is to just let the fight happen because this is the way of the goat and the goat herd.
It is actually very common for a doe that has recently kidded (or sometimes right before she kids) to try to upgrade her status/dominance in the herd in order to secure a higher status for her kids. There is nothing you can do to stop a fight such as this. Trying to keep does that want to fight apart will only prolong the inevitable because they will pick up the fighting whenever they get their next chance. The best thing to do is let them duke it out. It's brutal but it is the way of goats. It is their nature.
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You may also see other goats get involved and take sides in a fight. There is nothing you can do about this either. This is a gamble on the helper goat's part in hopes they will be the friend and helper of the winner thus helping their own status a bit (but not as much as actually fighting a fight).
Fortunately, once a winner is decided and status amongst the herd is clarified they will not fight anymore.