I just came across 〆 for the first time, used as an abbreviation of 締・閉:
Then found the vertical equivalent of 々 → 〻
Of course we’re all familiar with the prolonged sound mark ー in kana, which belongs to this category.
So I went investigating and found the following:
ゝ Repetition of last hiragana. More common in names: さゝき => ささき
ヽ Repetition of last katakana. トヽロ => トトロ
ゞ Repetition of last hiragana but adding dakuten.
ヾ Repetition of last katakana but adding dakuten.
Vertical writing:
〱 Repeat last kana.
〲 Repeat last kana + dakuten
〳
〵Repetition of last word.
〴
〵Repetition of last word + dakuten.
Edit: further info for anyone curious:
Can anyone confirm if they’re used all that often?
In print at all, or mostly handwriting?
In my experience, I used to see and use 〆切 all the time in excel spreadsheets, while working in a Japanese office (I was handling student enrolments and payments for Japanese classes).
So it may not just be a handwritten thing.
@LankyAusMan: You’re all set. For some reason, your profile hadn’t picked up on your ‘Level 25’-ness. It has been informed, so you’re no longer a ghost.