The owner must be very proactive in training their dogs. Also don't get a second dog so that your first dog has company. I prefer to keep a safe age difference between two dogs. If you get two similar age puppies even if they are from the same litter, they will grow together and try to show their hierarchy. If the dogs are poorly raised, they will develop behavioural issues.
You must see that the dogs are well raised and trained at the same time. A lot of times this happens because dogs tend to engage in rough games between themselves. The dogs feel it's the time to show who is more powerful. When a female is in her heat it triggers the fight more frequently.