(This is not a question about beds.)
Short and simple: has there ever been a ruling king or queen of some country that was a biological twin?
If so, I imagine this must have put some doubt on whether that monarch was really any more legitimate than their sibling.
The Man in the Iron Mask?
No… that was just fiction… 
Gp
Not once one brother cut off the other’s head.
The King was a twin, though his brother died at birth.
Oh, you don’t mean Elvis?
Actually, I seem to recall that, since twins had a definite birth order (like Kleenex, they pop out one at a time 
), the elder is the heir.
Too bad, he might have made a great Elvis impersonator :).
I don’t think it ever happened for the first born of a king, but I do remember having read once that according to french customs, in noble families, if twins were to be born, the heir was the second born (not sure why the second and not the first). I assume it would have been applied to the king’s heir. That say, I’m not sure that it’s true, but since, as uncommon as it could have been, such situation necessarily have happened from time to time in noble families, probably all countries had provisions in their laws or customs for these situation.
I suppose that there must be such provisions in the english law, since probably you can’t appoint both twins as Lords?
Wow, that must suck! “You could have been a king, but you came out 4 minutes after your brother.”
Mohammed Reza Pahlavi, the last Shah of Iran, had a
twin sister. However, this didn’t really cause any legitimacy problems since Shah was considered a males-only job.
I assume that the primogeniture rule would still follow-- I envision hasily attached toe-tags on the newborns. Is the root of the question the idea that one might be suspected/ accused as imposter? I think that when identical twins are born in positions of this kind of power, I’m sure people are careful about things. There are a lot of instances of twins in the genealogies I see on Google-- looks like someone worked it out.
Wasn’t the problem with with twins and primogeniture birthright the trouble with Jacob and Esau?
That thing about the French, though-- that’s odd, if it’s correct. Perhaps it’s based on the idea that since the twins rotate around in the womb the second one born is techically the oldest (which I have heard as a commonplace in some group-- I don’t think it has biological basis). I wonder if the royal families in Nigeria and Benin work with that rule, too, given the higher status given to the second twin that arrives?
You really think they do such legal heir-splitting?
I happens nearly constantly in fiction…
Only if they’re siamese twins.
Not because of any confusion about who was the elder. Esau was born first, and then traded his birthright to Jacob for a bowl of stew.