It's a bird!
It's a plane!
It's a Superman rumor!
Rumors are indeed flying that not only will the Man of Steel be making an appearance on 'The Flash' next season, but that Tom Welling will be once again playing Kal El / Clark Kent, about five years after 'Smallville' went off the air.
I wouldn't be surprised if it's only a cameo appearance, but it would be enough to confirm that the Last Son of Krypton does exist in the alternate dimension of this Flash. (And that would be the Comic Book Toobworld.)
A newcomer to Inner Toob might think that was a given, but if Welling does show up, he won't be the same superhero who ran for ten seasons on the WB and then the CW. That Clark Kent can be found in the Toobworld which is also home to 'The West Wing' and 'Mr. Sterling'.
The main reason that there has to be two mild-mannered reporters for a large metropolitan newspaper is due to the Green Arrow. In 'Smallville', the Green Arrow was played by Justin Hartley in over 70 episodes. That's too hefty a presence to be dismissed off-handedly. And the Oliver Queen played by Stephen Amell established an origin story and detailed personal life never hinted at in Tom Welling's show.
So because of the two Green Arrows, there must also be two Supermen. (When it comes to the CW, that is. We know there are plenty more in the greater Toobworld Dynamic!) But unlike the recastaways of Hartley and Amell, Tom Welling is both Supermen in 'Smallville' and in 'The Flash'.