A Chagrian with impeccable political skills, Amedda served as the Vice Chair of the Republic Senate in the chancelleries of Finis Valorum and his successor, Sheev Palpatine, who was secretly the Sith Lord Darth Sidious, something that Amedda was privy to knowing.
When Senator Palpatine succeeded Valorum as Chancellor, Amedda retained his position under the Republic's new leader. Amedda was one of the few in the galaxy who knew Palpatine was a Sith Lord, and assisted the Chancellor as Speaker of the Senate, helping him manipulate Senators into granting him more and more power.
Yoda knew and probaly told Obi-Wan. Darth Vader knew and possibly Thrawn if the book Outbound Flight is cannon. Also Tarkin might of known since he was friends with Palpatine before he became Emperor. Darth Plagueis.
It's unknown whether he ever told Darth Vader's true identity to his closest advisers; some of these knew Palpatine's identity as a secret Sith, so may have known about Darth Vader as well. It's possible Mas Amedda in particular was in the loop, but that hasn't been confirmed.
Vice Chair Mas Amedda was greedy, ambitious and cowardly, vices that Palpatine (Darth Sidious) detected early in his career in the Senate, he appealed to Amedda's ruthless desire for status in order to make him his right-hand man and help him topple Chancellor Finis Valorum.
How Mas Amedda Reacted to Palpatine being a Sith Lord [Legends]
What happened to Mas Amedda after Revenge of the Sith?
Amedda retained his position under the Palpatine administration, serving as one of the Chancellor's closest advisors during the burgeoning Separatist Crisis.
Shortly after the loss of the Outbound Flight project, Palpatine revealed to Amedda that he was, in fact, the Dark Lord of the Sith Darth Sidious. Initially crushed by this revelation, Amedda was persuaded to believe in Sidious's vision for a New Order and pledged his loyalty to the Sith Lord.
R2-D2 was present when Padmé gave birth to Leia and Luke, so he knew Luke was Anakin's son – but what he didn't know was that Anakin became Darth Vader. In addition to that, Anakin's fall was surely a painful event in R2-D2's life, one that he definitely wouldn't like to remember or even talk about.
Thus, while Qui-Gon was less powerful than the likes of Yoda and Mace Windu, Palpatine feared him more. In the simplest terms, he was a wild card, and with the Sidious executing an intricate decades-long plan to seize power and destroy the Jedi, a wild card was something he just couldn't afford.
A Sith Lord with the ability to cheat death and create life, Plagueis is the enigmatic mentor of Sheev Palpatine / Darth Sidious, who, in accordance to the Sith's Rule of Two, eventually betrays Plagueis by murdering him in his sleep, taking his place as Sith Master.
Following the proclamation of the New Order, the decree by which Palpatine transformed the Republic into the Empire, Mas Amedda was given the rank of Grand Vizier of the Empire on the Imperial Ruling Council.
Despite this ambition, Pestage was one of Palpatine's closest and most loyal servants, and was said to be one of the few people who truly knew him. He was entirely loyal and dedicated to his master, and Palpatine considered him to be the most trusted of his advisors and a good friend.
Maul most likely knew Sidious' double role as Palpatine and Sidious when he served as Palpatine's apprentice. He definitely knew about Palpatine's goal of creating a galactic empire ruled by the Sith. Maul knows that Sidious is the Emperor.
A Grand Moff received drastically increased authority, military forces, and funding as fitting their enhanced position. The position of Oversector governor was the sixth highest in the power structure after the Emperor, Supreme Commander, Grand Vizier, Advisors, and Grand Admirals.
Vader Outranked only by Sidious - CON: The only thing I can find that says Tarkin outranked Vader is a quote by David Filoni in 2015 during an interview about Rebels. He actually says that Tarkin outranks Vader.
He was the governor of the Outer Rim (Tarkin that is), but could he annul Thrawn's orders or did he outrank Thrawn. Tarkin probably outranked Thrawn since he was a Grand Moff (a political rank) whereas Thrawn was a Grand Admiral (military rank).
The sequence calls back to a similar scene in Revenge of the Sith, when Bail Organa orders Threepio's memory be erased to prevent him from sharing information about Anakin Skywalker's fall to the dark side.
Darth Vader acts like he doesn't even know C-3PO or R2D2. They were with him since he was a child. He did everything he could to forget his past. Also, his plating had been replaced many times, and protocol droids all look very similar.
Maybe R2-D2 didn't know Darth Vader was Luke's father because he didn't see Palpatine christening him Vader, or maybe he believed Anakin was dead because even Obi-Wan didn't know Anakin was alive.
Indeed he did. Dooku (aka Darth Tyrannus) knew his master's identity. At this point the Jedi thought Dooku was the Sith Master. The kidnapping was planned to reinforce public support for Palpatine while further confusing the Jedi about his true identity.
Like Master Yoda, Obi-Wan Kenobi learned Palpatine's true identity in Star Wars: Episode III - Revenge of the Sith. Where Yoda went after Palpatine, Obi-Wan targeted Darth Vader himself.
Originally Answered: How did Rex know that Palpatine was Sidious? Well, because he saw him. While Rex was obviously not himself while being controlled by Order 66, his eyes still functioned, so he saw who it was on the hologram that told him to execute Order 66.
