The episode where Dwight and Angela’s affair is revealed in a duel is “The Duel.”
It is the twelfth episode of season 5 and the 84th episode overall. Andy Bernard finally learns that his fiancée, Angela Martin, has been having an affair with Dwight Schrute. He challenges Dwight to a fight in the Dunder Mifflin parking lot, with Angela agreeing to honor the winner.
The duel does not resolve the love triangle. Andy uses his quiet Toyota Prius to surprise Dwight, then reveals that Angela had also been intimate with him. Realizing that Angela had deceived them both, Andy and Dwight abandon their claim to her. Andy cancels the wedding cake, while Dwight discards Angela’s bobblehead gift.
The episode aired on NBC on January 15, 2009. It is commonly confused with “Goodbye, Toby,” where the triangle’s earlier developments occur, but “The Duel” is the canonical title for the confrontation and revelation.