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All answers having "Childish comeback" as clue:

4 Character Answers:

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AMSO

ISSO



5 Character Answers:

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AMNOT

AMTOO

ARESO

CANSO

ISNOT

ISTOO



6 Character Answers:

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ARENOT

ARETOO



21 Character Answers:

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IKNOWYOUAREBUTWHATAMI