You don't even need to invoke the multiverse or alternate timelines/realities. This is possible in just our own one physical universe.

To the limits of our measurement, space-time has a flat (0) curvature. If it has a positive curvature, if you traveled in a straight line long enough, eventually you would end up back where you started. Such a universe would be without border but still finite in size. Negative curvature has its own properties. But to the limits of our measurements, the universe has a flat curvature.

What this means is that to the limit of our measurement, the universe isn't just large, but truly infinite. It literally goes on forever. Now, only a tiny fraction of that infinity is within our light cone. Most of this infinite universe is receding from us faster than the speed of light, so we can never interact with it.

But...a universe that is truly spatially infinite has some of the same properties as an infinite multiverse. If you could travel faster than light and survey an arbitrarily large volume, weird stuff would start to happen. Travel far enough out, and you'll find a copy of yourself. Down to the exact same genetics. Travel further still, and you'll find an exact atom-for-atom duplicate of the Solar System, including all the people currently living in it. There are an incredible number of ways to arrange the amount of atoms that make up a star system, but not an infinite number. And in infinite space, eventually you do get duplicates. Travel much further still, and you'll find an entire duplicate copy of our observable universe.

So, look up at the night sky. If the universe truly is infinite, there is another copy of you out there, born on an otherwise identical Earth, that is the same as you except was born cis. And if you had a fast enough space ship, you could fly out to meet them. In fact, there are an infinite number of copies of you.

Things get weird when you invoke true spatial infinity.

(Now, as far as just how far you would have to travel to find a duplicate Solar system, some quick searching turned up an estimate of 10^10^115 meters. The diameter of the observable universe is about 10^27 meters. "Astronomical" does not even begin to describe these distances.)