Why pp. to denote pages?

I got an interesting question from one of my students, so I immediately thought of the Straight Dope to help in finding an answer to her question. If anyone can come up with a reasonable explanation or direct me to relevent sources, I would greatly appreciate it.

Why is the abbreviation pp. used to denote pages (for example pp. 1-55) and how (and if possible when) did using pp. to abbreviate pages come into practice?

Plurals of single- and two-letter abbreviations are formed by doubling the last letter, for reasons I’m not aware of. This is one example of the phenomenon. Another is the annotation that “copies of this letter have been furnished to…” at the foot of a business letter, always written “cc:” (which does not stand for “carbon copy”). (The file copy of that letter will likely also have annotated on it those to whom copies were sent without so informing the addressee, annotated “bcc:” for “blind copies.”) Other examples may come to mind.

Plurals of single- and two-letter abbreviations are formed by doubling the last letter, for reasons I’m not aware of. This is one example of the phenomenon. Another is the annotation that “copies of this letter have been furnished to…” at the foot of a business letter, always written “cc:” (which does not stand for “carbon copy”). (The file copy of that letter will likely also have annotated on it those to whom copies were sent without so informing the addressee, annotated “bcc:” for “blind copies.”) Other examples may come to mind.

It’s a Roman custom; this method of forming the plural of an abbreviation by doubling the last letter was widely used at that time and can be found on many inscriptions and official text. For example was the highest Roman official during the republic, the consul, abbreviated COS, plural COSS.

Citing the following page in a text: f
Citing several following pages: ff

Thanks all!!