^I think that's his stock answer for transporter accidents, he said about the same thing in the Tuvix threads.
I think the one that returned to the ship was considered the "original" because whenever someone, anyone, beams up the others don't all say, " are you the original or is that copy on the surface the original" In other less silly words, you beam up, it's you. There's no doubt, it's still you. Also, when Thomas materialized, the people on the ship didn't know. So they were both
the original at that time. But then one continued on like normal and the other live a lonely and harsh time just trying to survive. To the prospective of his peers, Riker was just Riker, then the other Riker was discovered much later. Will never left and Thomas was marooned, except for the duplication, it isn't much different than if just another officer was marooned, no one would come back the same from that ordeal. So Will gets back first, therefore in the logic of this episode, he is the "original" although I reject that myself because by the same logic if both materialised on the pad, then they would both be original.
You really have to wonder about the transporter with episodes like this, Tuvix, Unatural Selection, and some others.
Some of the posts in the Tuvix thread implied that everytime the transporter beams someone it actually kills them and creates a copy.
Then in Unatural Selection they take a page out of the animated series book (Counter Clock Incident) and use it to fix everyone.
But, in it's basic function, the transporter is a disintegrator. But it does it carefully and with the intent to reintegrate the person at another destination. So the person is matter and is converted to energy to "beam" them to the destination. The transporter carefully stores a pattern to put the person back together from the energy of the original disintigration. What if you just keep adding energy? You could make copies of everyone. Need security? Beam up 7 Worfs, and they can all get knocked on their asses. I think there is a saftey of a sort to prevent that, but we all know computer programs can be modified. It's quite a subject, really.