Some persons born dual US-Canadian citizens at birth, particularly those born of even one Canadian parent on US soil and with no adult connections to or claims on US citizenship, may have a viable relinquishment option (as distinct from a renunciation option, open to almost everyone) for getting a Certificate of Loss of Nationality of the United States (CLN). Please see other threads of this and our partner website for a discussion of the difference between relinquishment and renunciation and why it might matter to you.
I learned yesterday by private email something I had suspected but had not been confirmed. Some duals-at-birth have, ideally after the age of 18, applied for an received a Canadian Citizenship Card from Citizenship and Immigration Canada at some point in their past. To get such a card, one has to provide evidence (in these cases) that at least one parent was a Canadian citizen at the time you were born, and you also must swear and sign an oath of allegiance to the Queen of Canada.