In the event of an unauthorized debit transaction, you should first contact your biller to try to resolve the situation. In general, you have 90 days from the date of the erroneous debit to ask for your payment back. However, you only have a right to have your payment returned if the debit went through on the wrong date, was for the wrong amount, or was for an agreement you had cancelled.
You can also try to get the money back directly from your financial institution. If this fails, you can complain about your bank to the Financial Consumer Agency of Canada.