B-1, r. 3.1 - Code of Professional Conduct of Lawyers

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87. A lawyer must not act against a former client in the same matter, in a related matter or in any other matter if, when acting for the former client, the lawyer obtained confidential information that may result in prejudice to that client or if knowledge of personal facts regarding the former client or the conduct of his affairs would provide the new client with an undue advantage, unless the lawyer obtains the consent of his former client.
O.C. 129-2015, s. 87.