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All answers having "Go back and forth" as clue:

3 Character Answers:

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PLY

WAG



4 Character Answers:

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PACE

SWAY

YOYO



5 Character Answers:

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ARGUE

WAVER



6 Character Answers:

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DEBATE

SEESAW

TEETER

TOGGLE

WAGGLE



7 Character Answers:

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COMMUTE

SHUTTLE



8 Character Answers:

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PINGPONG



9 Character Answers:

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OSCILLATE

TAKETURNS

VACILLATE