What could be worse a meteorite hitting you? Two meteorites hitting you, at the same time!
As shown in the above HiRISE image, this is exactly what happened on Mars. These two impact craters were formed simultaneously, but how do we know that?
If one meteor smashed into the planet, followed by another impact at a later date, one of the craters would overlap the other. But for this double impact to look so symmetrical, they had to have impacted at the same time.
Is this a case of simple luck? Did the cosmos decide to throw two space rocks at Mars into the same place at the same time? Unlikely. It was most likely caused by one object that split into two when entering the Martian atmosphere. That way, both halves (coincidentally of about the same size, in this case) impacted right next to each other, creating this fascinating double-impact crater, sharing one crater rim.
Interestingly, as noted on the HiRISE mission website, we know of some very oddly shaped asteroids and comets that could split into two when hitting a planetary atmosphere. Remember asteroid Itokawa (that was visited by the Japanese Hayabusa probe) with the double-lobed, "rubble pile" shape? The loose consistency of Itokawa would most likely cause it to disintegrate and break apart on hitting a planet, creating simultaneous impact craters.
Also, comet Hartley 2 with its signature "dog-bone" shape, could break into two, creating a double impact like this Mars example.
But why stop at a double-impact crater when you could have a triple-impact crater? Mars has one of those too! It's less defined as the double-impact crater shown above, probably because this triple impact crater is older, thus more eroded, but it's impressive all the same.