Duplicity means deliberately deceiving someone by pretending to believe or feel one thing while actually believing or feeling something else. It's a kind of two-faced dishonesty where someone shows one version of themselves to your face but acts completely differently behind your back.

Imagine a student who acts friendly toward a classmate but secretly spreads rumors about them, or someone who promises to help with a group project but intentionally does poor work to make others look bad. That's duplicity: pretending to be loyal or helpful while actually working against someone.

Someone acting with duplicity might smile and agree with you in person, then criticize your ideas to others later. They're playing both sides.

Duplicity is more calculated than simply changing your mind or making a mistake. It requires planning and persistence: the duplicitous person actively maintains a false front while pursuing a hidden agenda. In stories, villains often use duplicity to gain trust before betraying the heroes. In real life, duplicity destroys friendships and partnerships because trust, once broken this way, is nearly impossible to rebuild.