A Canterlot Wedding

Just as the Princess Cadence and Shining Armor are about to be married, Twilight Sparkle arrives in the nick of time alongside the real Princess Cadence, with the latter exposing the other Cadence as a changeling in disguise. In turn, the fake Cadence reveals herself as Chrysalis, the evil queen of the changelings.

Example of:
Confronting Your Imposter