🚀 go-pugleaf

Source: Hing, Handling Immigration Cases, John Wiley and Sons, 1985,
Chapter 12

These are the ways citizenship can be lost by anyone, no matter how obtained,

a) naturalization in a foreign country

b) granting an oath of allegiance to a foreign country.  This cannot
be a mere technicality as when a person promised allegiance to the
British Crown as part of joining the Canadian bar but otherwise
acted like and considered himself a U. S. citizen.

c) service in a foreign service.  Substantial intent is required here.

d) Employment of a foreign government in a significant post such as
legislator.  (This was what happened to Kanane when he was elected
a member of the Israeli parliament.)

e) Formal written renunciation of citizenship

f) treason

g) attempting to forcefully overthrow the US government.

People who have been naturalized as opposed to gaining citizenship through
birth or by various derivative methods can lose their citizenship.

a) the original naturalization was invalid (obtained through fraud)

b) a naturalized person who establishes a permanent foreign residence
may lose citizenship.  When one applied for naturalization, one had
to have stated that one intended to live permanently in the United
States.  If one leaves within five years, the burden of proof is on
the alleged citizen to show that he really did intend to stay in
the United States at the time he was naturalized.

c) joining a prescribed organization within five years, e. g.
Communist Party

Also one who gained citizenship by derivative means can also lose
citizenship if the original party was denaturalized.  I. E. children
of naturalized citizens can lose their citizenship under certain of the
above conditions.

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Note, that there is no formal mechanism to remove citizenship from someone.
However, if a foreign consular officer suspects that a person in the
country has done one of the above acts, they must report that.  The
Department of State then gives them a formal "certificiate of loss of
citizenship."  This determination can of course be appealed.

The issue will usually arise when a person attempts to return to the US.
Due to the laxness of US Canadian border, I suspect that the "dual citizens"
you refer to simply think they are still citizens of the US and so far
have not gotten an unpleasant surprise.  (If they have children in Canada,
then the children are going to see some problems)

I have heard of cases where a person has a dual citizenship because they
were the offspring of citizens of two different countries or were born
in the US and were offspring of citizens of a foreign country.

Also if someone is naturalized, although they must declare their unconditional
allegiance to the US, there is no procedural requirement that they renounce
their foreign citizenship.  Thus, if the foreign government does not consider
swearing unconditional allegiance to the US, grounds for renunciation of
the foreign citizenship, they appear to be able to hold dual citizenship.
(This latter statement was double-checked against the code of federal
regulations, title eight due to a mail discussion in which someone disagreed
with it.)