It's extremely rare. It occurs nowhere else in the solar system, and isn't even true for the vast majority of Earth's history and future. We could survey a million exoplanets and likely not find even one single other example.
Imagine an all-blonde beach party in Siciliy. Pretty rare right?
So is a beach party where everyone is tall, or short, or has the same name, or a beach party during which there's an earthquake, a meteor etc etc.
All would be rare events, but there are so many rare events that could possibly happen that is is not that rare to witness something rare, especially if we are looking for that.
Therefore witnessing a rare event may not mean much.
Neat, though.