Ok where is Sanji's twin gone?
If you mean Duval, he was unironic foreshadowing of Sanjis backstory, the "man in the iron mask" who was ostracised for reasons he could not help lol. Sanji is also based on the "Ugly Duckling" story who turns into a black swan in some versions IIRC and Duval was famous due to his unfortunate ugliness and resemblance to Sanji, being his ugly, "evil twin".
Duval will return by EOS for the final war I'm sure, Luffy is gonna have the BIGGEST army of allies by EOS, something like that infamous scene from Avengers Endgame but in One Piece. He's already an ally now like Bellamy is too for example ofc.
https://www.reddit.com/r/OnePiece/comments/4ugeed
Hey everyone! I've seen a couple comments here and there relating Sanji's childhood imprisonment to the Man in the Iron Mask story, but no concrete theories about what this could mean, so I have a few thoughts on it.
Though based on a real prisoner in the late 1600's who Louis XIV forced to always wear a velvet mask to hide his identity and who was made to serve other prisoners, one of the most popular versions of the story is that of Alexandre Dumas, translated into a film in 1998. The prisoner, now shown wearing an iron mask, was Louis XIV's twin brother who was locked away to avoid complications in Louis rise to power as king. However, after his reveal as Louis' brother, the twin proved to be the kinder and more effective ruler who took over in his Louis' place as King.
Tying this to the Vinsmokes, the story of the man in the iron mask comes with a number of similarities in all of its forms to Sanji's background and story, but most especially to his connection as a quadruplet in the Vinsmoke family. The kinder of his brothers, willing to serve others and help those in need, Sanji gets locked away wearing an iron mask. I believe that, if his story does link to that of the man in the iron mask, this could lead to him eventually becoming the true heir of the Vinsmoke family, overtaking his brothers in power and defeating his father by the end of the Whole Cake arc. We have already seen that he disagrees with his family's actions and dislikes them on a deeply personal level, and with the Vinsmokes rising as a major antagonist of the Whole Cake Island arc, he will need to face them eventually. While Sanji wouldn't want the position as head of the family, he would be the one recognized by the World Government and, with the Vinsmokes invited to Reverie, it would gain him an invitation to the World Council, essentially giving the Strawhats a way in as well.
A few side points to add extra connections and ideas:
"Iron Mask" Duval was initially introduced as a man wearing an iron mask so that others wouldn't recognize him as Sanji.
The Vinsmoke family already has some fairly strong French connections, Sanji especially.
In becoming the head of the Vinsmoke family, Sanji would also have control of the Germa 66, granting the Strawhats more allies in their eventual fight against the World Government. This could be especially useful as Germa 66 already has a historic connection to fighting the Marines.
TL;DR: Sanji's childhood is a parallel to The Man in the Iron Mask and at the end of Whole Cake Island, he will take his place as the head of the Vinsmoke family, gaining him and the Strawhats an invitation to Reverie, thus giving them a reason to go.
It's probably nothing too but Brook had the most hilarious reaction upon seeing Sanjis much uglier doppleganger in Duval and Sanjis "plastic surgery move" on Duval also foreshadowed what his family can do due to Judge's modifications, especially when Yonji had his face restructured/reshaped after Sanji fucked it up ofc lol.
Brook barely knew Sanji at that point but he had the strongest reaction to the Duval thing, kinda odd imo. But probably nothing indeed, I'm probs overthinking it.
Brook did drop casually that he knew of Germa and thus the Vinsmokes I'm sure back at WCI though and Oda never elaborated on how or why either. Brook is from West Blue but served as a convoy for some kingdom apparently, was not exactly "fond" of Germa and Oda gave him a LOT of shine in WCI, Sanjis personal arc too, aside from Baratie his origin arc ofc.
We also got actual "twins" of Sanji, identical siblings/genetic doppelgangers or w/e with the reveal of his identical brothers and sister too at WCI ofc.