Iirc Thomas is not Will’s copy. Basically the transporter has a malfunction where Thomas and Will are both constituted on a transporter pad. So both are the “original” and also both are neither. In a sense, the William Thomas Riker who stepped onto that pad that day ceased to be, forked into two different lives.

I used to use this episode to talk about identity when I taught ethics back in the day. Both men carried on their lives under the belief that they were the same man who stepped onto a transporter pad on Nervala IV. Their experiences defined who they were, demonstrating that it’s more than biological material that makes any one of us us.

Now the deeper question is: since you can’t add matter to the universe, what’s missing to give life to two Rikers?