YOU SAID:

fire on fire would normally kill 火の火は通常殺す

INTO JAPANESE

fire on fireは通常、火の火は通常殺すだろう

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fire on fire will usually kill fire

INTO JAPANESE

火の火は通常火を殺します

BACK INTO ENGLISH

Fire fire usually kills fire

INTO JAPANESE

火の火は通常火を殺します

BACK INTO ENGLISH

Fire fire usually kills fire