It is important to recognise that Marlow is not intentionally duplicitous. He never sets out deliberately to deceive either Kate or her father, Mr. Hardcastle. It is his curious shyness with women of his own social class that causes the confusion that forms so much of the comedy in this play. What Kate does to disentangle this confusion is to dress herself in the garb of a servingwoman, and to play the part of a maid in her household in order to "stoop to conquer" Marlow. Note how she describes her reasons for disguising herself in Act III:
But my chief aim is to take my gentleman off his guard, and like an invisible champion of romance, examine the giant's force before I offer to combat.
It is Kate, therefore, who is duplicitous, in the way that she intentionally disguises herself as being of a different social class in order to get to know the man her father wants her to marry because she realises after her first meeting with Marlow that, in her own identity, he is far too timid and shy for her to get to know him. It is only when she appears before him as a maid that she is able to gain a true measure of his worth as a character, and once he has declared his love for Kate as a maid, he is able to relate to her in her "normal" self to. Kate thus cures Marlow through disguising herself and tricking him, which also gives her the opportunity to get to know her future husband.