Without a malfunction killing you, how does the copy thing prove the transporter kills you? IMHO at best it proves it can work as a replicator.
The idea that a teleporter kills you is on the basis that the *only* thing the teleporter does is create a copy of you on the other side. It scans you, sends the information about you to the other end, and another you is created from matter that exists over there. At that point, all of the original you still exists on the sending end - so it kills you just as an afterthought. If it just didn't do that last step, the normal result would be two copies every time.
That isn't how transporters are implied to function in Star Trek, though. The implication is that the physical atoms of a person are sent through the beam to the other side and reassembled.
I think the accidental duplication of Riker is best explained as the result of error corrections. Some atoms may be lost in transport, so additional matter is injected in to fill in the gaps - using the context to fill in between. Normally this is less than 1%, but in an extreme case, as much as 50% can be lost and still produce a successful transport - because taking out half a random half of the atoms of a pattern still lets you infer the rest. So each copy got half of Riker's original matter, and the rest was inferred.
But in ST we SEE how you're FIRST disintegrated and THEN reintegrated in the destination.
We also see people conscious, talking and acting while in the process of transport.
We see Roga Danar able to "break out" of a transport beam. How the hell does that work?
In addition to the craziness of what it 'normally' does, it doesn't help that over the decades, how transporters work, and what they're capable of, is wildly inconsistent.