People were aware of this since we started talking of going there. Since it's going to be a one way trip and they likely won't be able to bring enough supplies to last for backup to get there. That's why most if not all the older astronauts said they would go and set up a base and greenhouse for the next group. They would willingly sacrifice themselves so the younger ones could survive.
A round trip would take about 21 months. It would take a minimum of 18 months for a resupply to show back up assuming that refuel and reload were ready for the ship upon return. But that's assuming we use a ship to carry it. It actually wouldn't be that hard to have 4 years worth of supplies on the first trip. Much of it can actually be sent ahead to be retrieved once they get there. They just send it like they did the probes. A controlled hard entry to make sure it's intact and viola, supplies are there waiting. Water would be the hardest supply to take but it's possible with current tech. However, no one has actually tried to live in an environment like Mars. It's almost certain that things will go wrong and probably at the worst moment. Space is not kind and will kill you at the slightest problem and Mars with it's much reduced atmosphere is not any better.
It's also worth considering if you send 50+ people on a year long voyage, even if you pick a bunch of ideal, healthy candidates, there are GOING to be some medical complications. That's literally unavoidable.
Between accidents, human error, human physiology, limited treatment options, and shear probability, someone will probably not make it there alive.
Between accidents, human error, human physiology, limited treatment options, and shear probability, someone will probably not make it there alive.