it comes from the latin practice of forming plural abbreviations by duplicating the last letter of the abbreviation. A manuscript is ms; more than one is ms. In the case of the words you listed the abbreviation in latin is just one letter: p. for page (pagina), c. for capitula.
Good question! The best explanation I can find online is from Wikipedia:
"Plurals are often formed by doubling the last letter of the abbreviation. Most of these deal with writing and publishing: MS=manuscript, MSS=manuscripts; l=line, ll=lines; p=page, pp=pages; s=section, ss=sections; op.=opus, opp.=opera. This form, derived from Latin, is used in Europe in many places: dd=didots."
But this isn't very satisfying because it doesn't say exactly how this usage derives from Latin or specify any Latin forms from which the abbreviations derive.
I did find a page of Latin abbreviations that says pp stands for "pluta paper," but I think this is almost certainly wrong. I can't find any other source for this, and as far as I can tell, neither "pluta" nor "paper" is a Latin word. Furthermore, "Pluta Paper" is a kind of cut-out paper doll book that was popular in the 1980s. (Of course that doesn't prove it isn't also a Latin phrase, but I'm suspicious.)
I would have expected more details to be easily found. I hope this helps a little.
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