TASK DEFINITION: You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
PROBLEM: Category: REMEMBRANCE OF THINGS "CAST" 
Clue: Hey, baby, you a Gemini?  These twin brothers of Helen of Troy are, literally

SOLUTION: castor & pollux

PROBLEM: Category: SIGNS & SYMBOLS 
Clue: Symbols of mystery, they're the mysterious doubles of the people who cast them

SOLUTION: shadows

PROBLEM: Category: OPERA 
Clue: In "Cavalleria Rusticana", an ear gets bitten when Alfio challenges Turiddu to their fateful one of these

SOLUTION:
a duel