That's an interesting question.
One of the discussions earlier was about whether the basis for triggering a review should be limited to new evidence. Obviously, if the evidence has already been considered, there could be a duplication of work. However, if someone wants to present new evidence, I think the commission should have the power to assess evidence that wasn't presented during the original proceedings.
Consider the argument that someone else committed the crime. If that argument wasn't presented at trial, I think it's appropriate that the commission would want to hear the witness in order to assess whether a miscarriage of justice occurred. If the witness was already heard at trial, the commission might not gain much from hearing the witness again.