In act III, scene IV, mistaken identity becomes more complicated when Sir Andrew Aguecheek challenges Cesario (who is really Viola) to a duel. Viola is ready to admit that she is too weak to fight but eventually has to face Sir Andrew. This plot twist highlights how Viola’s decision to dress as a man has some potentially dangerous consequences. However, Antonio suddenly appears on the scene and, thinking that Viola is Sebastian, comes to the aid of the person he thinks is his friend.
No one is able to understand how Cesario happens to be Antonio’s friend. When police officers intervene and recognize Antonio as a wanted man in Illyria, Antonio gives his reasons for fighting. He explains that that he is trying to save the young man he once saved from drowning:
ANTONIO: Let me speak a little. This youth that you see here
I snatched one half out of the jaws of death,
Relieved him with such sanctity of love,—
And to his image, which methought did promise
Most venerable worth, did I devotion.
Antonio then asks Cesario/Viola to give him back his purse, which he lent earlier to Sebastian. She has no idea what he is talking about, of course, but Antonio, still mistaking Viola as Sebastian, thinks Sebastian is betraying him.
The characters in the scene are befuddled and unable to comprehend these happenings. But Viola understands that this case of mistaken identity hints at the fact that her brother Sebastian is alive and has been in the company of Antonio in Illyria:
VIOLA: Methinks his words do from such passion fly
That he believes himself; so do not I.
Prove true, imagination; O prove true,
That I, dear brother, be now ta'en for you!