The Adolf Hiter Doppelgänger is a person who resembled Adolf Hitler.
When the Hitler doppelgänger was walking towards his car he got a call from Bart, who was trying to figure out if the water went clockwise in the Southern Hemisphere. When he went to answer the call, he was too late, and he missed the call. Then a man passes by and salutes him.
This man is most likely not the real Hitler, as Hitler has been mentioned to be dead in the series.