The Penultimate Peril
Throughout The Penultimate Peril, the Baudelaires have trouble telling him and Ernest apart. When Dewey was killed, Frank or Ernest placed the Baudelaire children in Room 121 while the other one placed Count Olaf in Room 165.
During the trial of the Baudelaire children and Count Olaf, they submitted a Commonplace book as evidence.
When the Hotel Denouement Fire was started, the Woman With Hair But No Beard directed them towards the elevator. When Violet Baudelaire shouts for them to call the fire department, they quote "Which one" as the elevator closes on them.
It is unknown if he survived the fire that destroyed the Hotel Denouement.