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All answers having "So long!" as clue:

3 Character Answers:

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BYE

RHO



4 Character Answers:

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AGES

CIAO

TATA



5 Character Answers:

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ADIEU

ADIOS

LATER

SEEYA



6 Character Answers:

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BYEBYE

BYENOW

SEEYOU



8 Character Answers:

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TOODLEOO



10 Character Answers:

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IMOUTAHERE

SEEYALATER



12 Character Answers:

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HAVEANICEDAY