One fundamental way of testing the credibility and reliability of a witness in a trial is by demonstrating that the witness has said something different on a previous occasion. 

In the event of a retrial, the testimony of a witness at the first trial becomes a valuable potential source of material for the purpose of impeachment. This point was recently reiterated by Paciocco, J., speaking for a unanimous Court of Appeal in R. v. Morillo, [2018] O.J. No. 3405, at para. 20:

When Mr. Morillo sought to confront the officer with inconsistencies between his testimony at the retrial and his testimony at the first trial, the Justice of the Peace refused to allow it. She apparently laboured under the misconception that since retrials are to be determined on their own evidence, no use should be made of testimony taken at the prior trial, even to demonstrate inconsistency. If this is so, she erred in law. It is trite law that prior inconsistent testimony from a first trial can be used to impeach a witness at a retrial.

Other considerations:

Paciocco, J. in Morillo, at para. 26, also discusses the implications for cross-examination if a party does not have a transcript of the testimony of a witness from the first trial:

...A party need not have a transcript to cross-examine a witness about their prior inconsistent testimony. …If the witness agrees they made the prior inconsistent statement, the contradiction is established. The risk in not having a transcript is that if the witness denies making a prior inconsistent statement when asked, that denial cannot be contradicted and hence the contradiction cannot be proved. Put otherwise, Mr. Morillo would have been well advised to have had the transcript of the first trial with him for use in cross-examination, but the absence of a transcript does not prevent him from cross-examining the officer about the contradictions he believes to exist.

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