I accidentally filed my taxes twice, what do I do? Can I delete one or will they delete it or how do I let them know one was an accident?

Yes, you'll need to contact the CRA. 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