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I am sorry you are having trouble. This is a very common problem. It occurs because the cat that went to the vet came home with smells from the animal hospital on her fur. This is very scary to the cat that stayed home because she thinks you brought in a srange cat.
Rub a damp cloth all over that cat that stayed home. Then rub it all over the cat that went out. Repeat this several times. Soon they will recognize each others scent and be friends again.
Please let me know how things are going.
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