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In Star Wars: Revenge of the Sith, Chancellor Palpatine tells Anakin the tale of Darth Plagueis the Wise. He told Anakin that if he joined the dark side he could save the people he loved from death by extending their midichlorian count, just as Darth Plagueis had done (obviously more subtly than that).
When Anakin asked what had happened to Plagueis, Palpatine says:
Unfortunately, he taught his apprentice everything he knew. Then his apprentice...killed him in his sleep.
It is revealed (not in the movie) that it was Palpatine himself that was Plagueis' apprentice
After Anakin and Palpatine kill Mace Windu, Palpatine says,
To cheat death is a power only one has achieved, but if we work together I know we can discover the secret.
This implies that Palpatine doesn't already know the secrets but if Plagueis taught him "everything he knew" then surely he would have.
So did Sidious know the entire time or was "everything he knew" an exaggeration?
Just to be clear when I say "cheat death" I only mean extending someone's midichlorian count, not "transfer essence" or anything else like that.