Under the right circumstances, you are allowed to become a citizen of both Canada and the United States, simultaneously. Many people enjoy the benefits of dual citizenship, allowing them to to travel back and forth freely, vote, and otherwise take advantage of the rights of citizens in both countries.
Most U.S. citizens, including dual nationals, must use a U.S. passport to leave and enter the United States. Dual nationals may also be required by the foreign country to use its passport to enter and leave that country. Use of the foreign passport does not endanger U.S. citizenship.
Being a citizen of the United States, it is highly advisable to enter the United States on a US Passport.
That being said, you would have had to initially depart the United States using your US Passport.
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