Forbes The big worry, of course, is gravitation, which can ruin a dual orbit in one of two ways: either a gravitational interaction can “kick” one of the planets very hard, either sending it into the sun or out of the solar system, or the mutual gravitational attraction of the two planets can cause them to merge, resulting in a spectacular collision. by Ethan Siegel
'This latter case is, in fact, something that happened to Earth when the Solar System was only a few tens of millions of years old! The collision resulted in the formation of our Moon, and very likely caused a major resurfacing event on our planet.
'Two planets don’t do a great job of occupying the same exact orbit, because there’s no such thing as true stability in these cases. The best you can do is hope for a quasi-stable orbit, meaning that while, technically, on infinitely long timescales, everything is unstable, you can obtain configurations that last billions of years before one of these two “bad” things occurs.
'The unfortunate news, at least for now, is that out of the thousands of discovered planets around other stars, we don’t have any binary planet candidates yet. (You may have heard of one a few years ago, but it was retracted.) Of course, our technology hasn’t progressed to the point where we’ve discovered moons around exoplanets yet, either, and yet we fully expect them to be there.'