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All answers having "Simultaneously" as clue:

5 Character Answers:

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ASONE



6 Character Answers:

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ATONCE

UNTRUE



7 Character Answers:

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ADAMANT

ATATIME

ATONEGO



8 Character Answers:

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INUNISON

TOGETHER



9 Character Answers:

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ALLATONCE

ATONETIME



10 Character Answers:

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INPARALLEL



13 Character Answers:

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ATTHESAMETIME



15 Character Answers:

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INTHESAMEBREATH