Consider this one. Riker was split in a transporter accident, and the result was TWO beings. I've never heard anyone argue that both of them were not real people with actual "souls" or whatever makes a person a person (at least in the fictional world) - so one person became two.
The only reason "WIll" Riker remained Will is because he was the one that had led a "life" while the other one had remained trapped. Thomas wasn't any less WIlliam T Riker than the one we know simply because he had a "copied" body.
FWIW: Thomas is of Aramaic origin and means twin. Also, the TNG staff actually toyed with killing Will and replacing him with Thomas. Frakes was even warm to the idea. But it would have meant a demotion to Ops while Data was made First Officer.
That would have made for some interesting storylines. Does Deanna fall in love with this Riker?