correction/calrification:
Tell me, what law did congress pass after the 14th & the expatriation act(1868-1884) which clarified that the US did in fact accept as law the concept of dual citizenship. And for tht matter, why even the need for the expatriation act if dual nationality was in fact part of the law of the land?
None, because IT DOES NOT MATTER. The US will not recognize dual citizenship; just because another nation may attempt to lay claim to you does NOT affect what the United States holds your citizenship to be.
How does a person lose their US citizenship? The relevant means here would be willfully accepting the citizenship of another nation. It is an ACT by the person, not by the other nation, that changes your US citizenship. If Britain, or Canada, or Venezuela claims you as a citizen it does not cancel your US citizenship! Only your willful acceptance of such claim would affect your US standing.
Another nation can lay claim to you as it wills; it does not affect your standing here. The US considers it immaterial. I don't know how much more clear this can be.