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She vowed never to do such a thing again.
Literal
She [topic-は] [never-again-二度と] such thing [object-を] will-not-do [quotative-と] vowed.
二度と...ない is the 'never again' pattern, where 二度と always pairs with a negated verb. The vow's content is reported via quotative と + 誓った. そんなこと ('such a thing') is a discourse pronoun: it picks out something previously mentioned or contextually clear — the implicit referent of what she's vowing never to repeat.