The joys of time and space travel
From the 20th chapter:
Syaoran ran over to check on the Princess Sakura while Sakura Kinomoto did likewise to Syaoran Li.
"He looks like he's asleep but I can't wake him up!" said Sakura, then she looked over and saw the Syaoran from another world. "Kero, look over Syaoran while I talk to that guy please?" Mokona told Syaoran "Sakura's sleeping. I'll keep an eye on her." Syaoran walked to where the 'other' Sakura was and the two stood there staring...like lost friends but at the same time...complete strangers to one another.
Fai looked at the two. "Just as the time space witch told us...we may meet other versions of people we know in these other worlds but the Princess Sakura in this world...the resemblence is uncanny indeed."
As with many stories about many things using the concept of travelling through time, space and different dimensions, each writer has their own take on it, whether it's Doctor Who and The TARDIS, Star Trek using the sun to slingshot around the sun to travel back to save the whales, Flash using a 'Cosmic Treadmill' to run in place and vibrate to another dimension...well, you get the picture.
In the world known as The Arcademan Universe, my theory on time travel is based both on science and magic. The scientific aspect has to do with a
TACHYON, a sub-atomic particle which travels faster than the speed of light, thus enabling the user to go back through time. Mystics can control its use for dimensional purposes, since any device that allows time travel needs large amounts of energy. (As an example, Sakura used The Return Card to go back to meet Clow Reed bu required more energy than even she had so she used the power concealed in the shrine).
As far as someone existing back in time, I use the notion that the same person, no matter how close or different they are, cannot co-exist in the same dimension at the same time without major reprocussions. The person with the strongest willpower will be the dominant life form, which is why Princess Sakura and Syaoran Li both fell into a coma when the doppelgangers met each other. Hope that helps clears things up with the space-time continuum