there are a handful of ranges in IPv4 that are NOT globally unique. You're describing a much smaller set of IP ranges that are designed to be used+reused but not routable.
if you take someone elses public IPv4 address and they're using it, neither one of you will be functional, and they will come knocking on your door.
if you take someone elses public IPv4 address and they're using it, neither one of you will be functional, and they will come knocking on your door.