We have three Batgirls now and have nearly always had several Flashes and Green Lanterns. The Trinity I get, but the other identities?
Barbara and Kate have a lot more to differentiate them than some redundant characters. I think it's fine. Hell, I'd argue Barbara has had more historical claim to it than Kate who only became Batwoman because writers weren't going to let Barbara stop being Batgirl until she became a prop to hurt her dad with.
Oracle is one of DC's best examples of turning something terrible into gold, but she probably would have become Batwoman long before Killing Joke had the times been different.