As usual, it depends on what aspects of behavior you consider important. As an obvious example, if the two produce (or store) distinct structures that are (==), that should be fine. Similarly, if the effects are different in a way that does not matter to your application (e.g., one may buffer more input than the other, or they may read from the same files in different orders, or they may produce the same images on screen using different graphics operations) then that should be fine too.