Activity section on Common App?

I have had two leadership positions in a club, one of them being as a “PR officer” my junior year and then becoming “Vice President” my senior year. Should I bother mentioning anything about the previous officer position in my activities section? Would it be redundant? I have other activities to list so I’m not necessarily desperate to load my resume.

Also, if I want to list this previous position, would it be possible to do it while only having it show as “one” activity for that section rather than having to list them as two separate activities?

It’s fine to list both.

Yes. e.g.
PR Officer (11), Vice President (12)

It should be listed as one activity with both the positions listed. Otherwise, it would have an adverse effect as your dedication would be splitted, it appears padding, and you ask the adcom to read more unnecessarily.