The U.S. discourages dual citizenship because it can cause problems assisting you in a foreign country if you are a dual citizen. However, you will not use your U.S. citizenship unless you renounce it voluntarily or you were found to have been expatriated. You should continue to enter the U.S. on your U.S. passport.
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The US requires that you relinquish all prior allegiances to other countries. If Canada requires something similar, you might be violating that requirement. I would recommend speaking to an attorney who specializes in Canadian immigration law.
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You can be a dual citizen, but it is discouraged. The U.S. Embassy in Ottawa has an information post on this, see the link below.
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