It's not all about their appearance. How a cat moves and vocalizes i.e. its personality, can serve as obvious indicators in determining which cat is which.
Very much agree with this!
I actually got asked this question a couple of days ago in the offline world - I was mentioning that our shelter had lots of black cats, and a friend asked me how I could be sure the cat I had adopted was Bentley.
I was kind of startled by the question, because... well, it just wouldn't have been possible for me to adopt the wrong black cat, and it wouldn't have occurred to me to wonder that. They certainly aren't all identical. They have very different faces, builds, ways of moving, vocalizations, eyes, coat length/type, etc.
We do have a couple of pairs of cats at the shelter who look very similar, but of the ones I've worked with there are always small ways to tell them apart once you get to know them. I'm sure perfectly identical twin cats exist, but I also think they're a
lot more rare than people would assume at first glance.
Another thing about Bentley is that we knew each other before I adopted him. Even if I was blindfolded, the cat that took a flying leap into my arms and gave me a hug would be Bentley. A random cat wouldn't do that. Once you get to know them, you know their behavioral cues.
I don't think it would be impossible to have cats there would be truly no way to tell apart, but I think it would be exceedingly rare.