From Wiktionary, the free dictionary
〴 | U+3034, 〴
VERTICAL KANA REPEAT WITH VOICED SOUND MARK UPPER HALF |
|
- /"\ (/"\) — in horizontal digital transcription
〱 + ゛.
〲
- (in vertical text, dated) An iteration mark denoting the repetition of the previous two to four kana, with a dakuten applied to the first kana (denoting that it is voiced).
Do not confuse it with the hiragana ぐ (gu). The kana iteration mark is twice as long in vertical text.