Formal:
* The politician's promises were riddled with duplicity, masking his true intentions.
* The detective uncovered a web of duplicity in the case, with multiple suspects hiding their involvement.
* The CEO's duplicity in his dealings with the company's investors eventually led to his downfall.
Informal:
* I couldn't believe her duplicity - she was saying one thing to my face and another behind my back.
* He played the role of the loyal friend with such duplicity that it was hard to believe he was the one who stole my wallet.
* The movie's plot hinges on the protagonist's duplicity, as he pretends to be someone he isn't.
Remember that "duplicity" implies a deliberate intention to deceive, so use it carefully.