Code Geass R2 - 13

Something's bugging me, regarding the last episode of Code Geass. When Jeremiah is with Lelouch in the station, Orange's asking Lelouch "why did you turn against your own father" or something like that and Lelouch answers with "because I'm Lelouch vi Brittania!" and that is supposedly his justification. The thing is, when Lelou says that, Jeremiah gasps as if he didn't know that Lelouch is one of the sons of the Emperor. I mean, if he knew/knows who Lelouch is, then he knows something about his motives. The only explanation that I think of, for this to make any sense, is that at least two of the Emperor's sons are called Lelouch. Any ideas?

I think Lelouch's resolve surprised him.

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