You’ll need to introduce them slowly
You’ll need to introduce them slowly, in a way that doesn’t threaten anybody’s territory (keeping them in different parts of the house for a while, then introducing them on either side of a closed door is a common way to do it). You’ll also want to take the new cat to the vet and make sure it doesn’t have anything contagious. There are some scary diseases out there that cats can spread to each other pretty easily.
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