Now and again, Jean-Baptiste speaks of "the fundamental duplicity of man." This is an interesting word, duplicity. (Remember that we’re dealing with a translation here, but both the O’Brien and the Buss translation use the same word in English.) Duplicity means deceitfulness, but it also means double-ness. When you look at it that way, "duplicity" seems to crop up all over the place in The Fall. Jean-Baptiste is deceitful in the sense that he’s tricking you. He may be a liar, he’s certainly deceptive, and he’s admittedly "a play-actor." (And he does indeed play a lot of parts – just read his "Character Analysis" for the details. Each role is interesting in itself, but the fact that he has multiple roles at all is worth talking about. It means he is pretending, shifting, changing, and in all probability, lying to you.)
Jean-Baptiste is also "double," in the sense that he is not what he seems. He claims to be both a judge and a penitent, both every man and no man at all, both himself and you. He even calls himself "a double-faced Janus." When Jean-Baptiste speaks of duplicity, he’s referring not only to deceit and double-ness, but also to his own hypocrisy – arguably a marriage of the two. He says, "Modesty helped me to shine, humility to conquer, and virtue to oppress. I used to wage war by peaceful means." See the duplicity here?
Now that’s all good for Jean-Baptiste, but what does that have to do with the rest of men? What makes the duplicity fundamental, and what makes it common to all mankind? Remember that Jean-Baptiste’s confession is supposed to be yours, and that he’s painting a picture, as the epigraph would have us believe "of the vices of [a] whole generation." Jean-Baptiste may be hypocritical, double, and deceitful, but we all share the duplicity we condemn in his character. We all lie to ourselves, pretend our motives are pure, put on masks and play roles while we call it living. Or at least, that’s one interpretation of The Fall.