I haven't ever had a problem, but I've only ever added kittens to adult cats and the adults usually just hiss a bit then get over it, the worst we've had is our cat Elvis who usually bats them with his paw to let them know who's boss. They always get over it in the end. I would just let adult cats sort it out themselves, one will be less dominant than the other just keep an eye on them so they aren't actually doing any damage to each other. I do think if your adding a cat and you have a choice it is easier to choose a kitten, the adults seem to accept a kitten better.

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