
The canon novel Tarkin does indicate that it is public knowledge that a duel took place, so it's possible that someone directly observed a duel: I'm not aware of any source which indicates that anyone filmed the duel.

Is there anything in the Disney Canon or Expanded Universe about anybody observing the duel? Just imagine the news headlines and videos posted all over the galactic internet the next day. It would be enough to make people think Palpatine was a Sith Lord.

They would see the key moments leading up to the end of the Jedi. (After all, various governments, news agencies, and corporations have cameras and microphones aimed at embassies and government office buildings all over the world here on Earth today.) And there are hundreds (thousands?) of air cars flying within a few kilometers of the office. I could imagine hundreds of cameras secretly watched every corner of the Senate office building day and night. Anybody in a nearby building could see into the office and watch the final moments of the duel.
