but then I started to wonder if I was a dupe.

As I re-read Chpt 51 with Mr Knightly's reading of Frank's letter, though, I see that even Mr K (!) believes Frank is not dissembling as he says to Emma: "Playing a most dangerous game. Too much indebted to the event for his acquittal. No judge of his own manners by you. Always deceived in fact by his own wishes, and regardless of little besides his own convenience. Fancying you to have fathomed his secret. Natural enough! his own mind full of intrigue, that he should suspect it in others."