It is, but generally speaking due to tax treaty you're not double taxed. All your US taxes paid are credits in Canada. The old loop hole used to be that if you

It is, but generally speaking due to tax treaty you're not double taxed. All your US taxes paid are credits in Canada. The old loop hole used to be that if you were (Or intended to be) non-resident for 2 years or more you didn't owe taxes to Canada (you would need to do a deemed disposition of all your assets for tax purposes though). But then you couldn't vote either...