You should not file tax returns using two different SINs. If the Canada Revenue Agency (CRA) issues you a new Social Insurance Number (SIN), then your previous SIN is no longer valid.
We recommend you contact the CRA and explain the situation you have encountered so they can help you resolve the issue. If you are in Canada or the United States, you can call the CRA at 1-800-959-8281. If you are elsewhere, you can call 613-940-8495. This page has more ways to contact the CRA: Contact the Canada Revenue Agency
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