We may place humans on Mars for periods of time, perhaps even lengthy periods but the idea of an alternate earth is in fact ludicrous. We seem to have a hard time just managing the planet we're on.
Why is it ludicrous - do you have a specific issue that you object too? How is living on Mars very different from e.g. the international space station or conditions in a nuclear sub in terms of human habitats? The other bits seem relatively easier than the problem of getting people to the planet. In general I think arguments of the form "it can't be done soon because of resource constraints" are very weak - there are plenty of examples of people making such arguments and being proven wrong a couple of decades later.