The ancient Greek philosopher Heraclitus posutaletd that nothing in the universe is permanent-except for change. he discussed the parable of a man walking into a river=the water flows past, and the event is unique=nothing in that moment will ever be EXACTLY the same again. Is this correct? Will events in human history never recur? Or will someone named RALPH124c , eons from now, sit down at akeyboard and type this message? 
I’s entirely possible that in a parallel universe somewhere millions upon millions of light years away, there is a guy named ralph124c who looks exactly like you and posted the exact same OP on a message board exactly like this one at the exact same time that you did.
Except that the distance between this universe and a parallel one could not be measured in any unit of length, not even the light-year. Could it?
According to the special report put out by Scientific American a few months ago on parallel universes, the nearest identical parallel universe is somewhere in the neighborhood of 10^10[sup]118[/sup] meters away. Not only would there be a doppleganger of you in this universe, but he would live on a planet called Earth and be reading a thread on a message board called the Straight Dope about parallel universes asking whether or not the distance between his universe and ours could be measured in any unit of length.
How can anyone possibly know this?
Theoretical physics, I imagine.
Except that it’s impossible for those two to be identical. We know from experience with super heroes that your doppelgänger is left handed. And all left handed people on earth are thus the other universe’s doppelgängers stranded here.
There’s a finite number of ways that matter and energy can be arranged in a finite volume. Since the universe is infinite some arrangements must repeat.
If the moment is repeated, how would we know? wouldn’t we just be there not knowing the difference?
A related thought: imagnie that at the end of each year, you put together a DVD “of the year”, containing highlights of what happened that year in politics, culture, government, pop culture, etc.
Well, there are only a finite number of possible DVDs.
So eventually, a year would have to pass that was so similar to a previous year that its highlights DVD was precisely identical.
If I start counting out loud 1, 2, 3, 4, …
I’ll never say 1 again.
How do you know the universe is infinite? Certainly it has no edge, but neither does the surface of a sphere.
But if you were immortal, you’d eventually get to a point where the numbers were larger than you could practically remember/pronounce, even with any imaginable constructable aid, and you’d stop being able to say unique numbers.
In other words it’s just speculation, but with consistent math.
Is the universe infinite? It hasn’t existed for an infinite time, why should it be infinite in extent? I thought it was finite but unbounded, like the surface of a sphere, as Brain Glutton says. And even if it is infinite in extent, who says there’s only a finite way matter and energy can be arranged? Reality doesn’t have to conform to our expectations.
The article didn’t actually say that the existence of parallel universes was proven, only that certain observations of the universe in certain parts of the electromagnetic spectrum were consistent with a theory about parallel universes. More investigation is needed. However, the question is one that almost by definition can’t be settled definitely.
until you reach twenty-ONE. 
That was my (admittedly limited) understanding of it.
What if our finite universe is floating out there in infinite empty space? Wouldn’t that allow for other universes?
Yes. But reason do we have to believe that’s true?
And has a goatee.
Nothing really, except that it’s neat to think about.