adverb
Only when written 努努 / 努々
Used in expressions of prohibition, often with ～な or ～まじ; literary and somewhat archaic.
ゆめゆめ忘れることなかれ。
You must never forget.
adverb
Used with negative verb forms; emphasizes complete absence or prohibition. Literary and rare.
ゆめゆめ疑ってはいけない。
You must not doubt even a little bit.
adverb
Only when written 努努 / 努々
Rare adverbial use meaning 'diligently'; mostly found in older texts.
努努励めと師は言った。
The teacher said, 'Work diligently.'
The historical derivation is uncertain. The kanji 努 (effort) and 夢 (dream) are used as ateji for the reading ゆめゆめ, which may be a reduplicated form of ゆめ (dream) or an emphatic expression.