Kana iteration marks are are rarely used today, and hence there is not much online information on it besides this Wikipedia article, which leaves me with a couple questions.
Is there any kana that cannot be expressed with an iteration mark? What about digraphs used in yōon, long vowels, or ん and ン?
Can a kana with handakuten such as ぷ be expressed with iteration marks? Presumably, ぷゝ with an unvoiced mark would be equivalent to ぷふ, but what about ぷゞ with a voiced one? Would it mean ぷぷ with handakuten or ぷぶ with dakuten?
Hoping someone will be able to help me better understand these unique symbols!
Mora with a glide: I guess, if it is ever used, the mark will express the whole mora rather than just the glide because two consecutive glides are not permitted in Japanese phonology.
Long vowel: For katakana, there are specialized symbols ー and ｜, so you cannot use it. For hiragana, you can you it.
Nasal coda: Japanese does not allow two consecutive nasal codas (in the native stratum), so there will not be such situation with hiragana. For very artificial onomatopoeia written in katakana, you might be able to use it.
Other than that, I think there is no restriction in using the mark as long as that sequence appears.
ぷゞ will mean ぷぶ. In hiragana, the combination ぷぷ should not arise. Japanese phonology for the native stratum does not allow it. When used in onomatopoeia or gairaigo, expressed in katakana, I think you cannot use the mark, and have to simply write ブプ.