> when `b` has same values of `a` before it is mutated.

There might be no such object. Or it might exist, but you still wouldn't want it.

Consider the case when a is a coroutine that has just started (a=coro()). You call f with a, and then advance a to the next yielding point. Is f called with a? You say no. Ok. But imagine you call coro again, and call that b (b=coro()). Is f called with b? Do you really want to say yes to that?