http://www.amendment-13.org/Opinions?
The "missing" 13th Amendment to the Constitution of the United States
reads as follows:
"If any citizen of the United States shall accept, claim, receive, or
retain any title of nobility or honour, or shall without the consent of Congress, accept and retain any present, pension, office, or emolument of any kind whatever, from any emperor, king, prince, or foreign power, such person shall cease to be a citizen of the United States, and shall be incapable of holding any office of trust or profit under them, or either of them."
The controversy falls to whether or not Virginia ratified the amendment. Up to 50 years later, the amendment was printed by states with today's 13th amendment listed as the 14th.
THIS YEAR, New Hampshire has a bill introduced to re-certify their ratification of this amendment. If Virginia introduced a similar bill, that it re-certifies their ratification, it could arguably put-in-force the original 13th Amendment.
Anyone knighted would therefore be stripped of citizenship and barred from office. Some proponents argue more extreme consequences such as it barring all lawyers from serving, although I am not convinced of such an argument.
I am convinced that the issue is unsolved, and that they're arguments that it is in fact ratified are convincing.
(This amendment would not, of course, overwrite or affect the current amendment known as the 13th)