You cannot step into the same river twice.

You cannot step twice into the same river, for other waters are constantly flowing on.

No man ever steps in the same river twice, for it's not the same river and he's not the same man.

You cannot step twice into the same river; for other waters are continually flowing in.

It is not possible to step twice into the same river

You can never step into the same river; for new waters are always flowing on to you.