It doesn't matter if the imports come through the U.S. or come direct from China (or wherever), the goods follow the "Rules of Origin" outlined in NAFTA.  So if something is manufactured in China and imported to the U.S. and then into Canada they still face Canadian duties on Chinese-made goods.

The only exception is if they pass the 'content' requirements.  For example a car that is mostly made in the U.S. but uses a few bits of electronics imported from China would still be considered to be made in America and those bits of electronics would only face U.S. duties on their original import, not additional Canadian duties when the whole car was imported into Canada.

And as for "free trade in general", we don't have free trade with China so I don't know what "in general" applies here.  Is it the free trade with have with several European nations that allows more imports from China?  Free trade with Chile?  I guess that one is spelt almost the same as China?