This was a question that's always given me a headache until I got it right. The thing is, while it is possible (as in the chances are not zero), the chances you have are just so low and close to 0 you'd probably have more luck winning a lottery multiple times instead.

Even if you had the astronomically low chance of generating an already used privkey and somehow managed to be the one to do it, an used privkey does not necessarily mean it also has a balance on it.

If someone ever happens to be "lucky enough" to generate the same key as someone else who still has it as an active wallet, the best thing they can do is just leave the address alone once they find out and move their funds to another wallet. Otherwise, taking someone else's funds has a simple definition: theft.

Many things have a probability, but it doesn't mean they will also ever happen/exist. Generating two identical privkeys is one of them.