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All answers having "On the double!" as clue:

3 Character Answers:

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NOW

PDQ



4 Character Answers:

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ASAP

CHOP

FAST

STAT



5 Character Answers:

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APACE



6 Character Answers:

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ATONCE

MOVEIT

PRESTO

PRONTO



7 Character Answers:

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SWIFTLY



8 Character Answers:

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CHOPCHOP