What does a US citizen need to return to the US from Canada?
Entry into the United States: When traveling by air from Canada, U.S. citizens must present a U.S. passport book or other approved identification document.
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There are no penalties. The customs officials might be furious but the only thing they can do is confirm you're citizen and let you back in. As for leaving the US, the US doesn't have exit checkpoints so you can leave on any passport.
You can travel abroad for as long as you'd like without any risk of losing your U.S. citizenship. And if you plan to stay outside of the United States for longer than a year, you won't need a re-entry permit in order to return, as is the case for green card holders (permanent residents).
U.S. nationals, including U.S. dual nationals, must use a U.S. passport to enter and leave the United States. U.S. dual nationals may also be required by the country of their foreign nationality to use that country's passport to enter and leave that country.