Decide on return of dedup_citations() when manual = FALSE

[LukasWallrich]

Currently, dedup_citations is documented as always returning a list, yet it returns a df when manual = FALSE. That needs to be aligned - @kaitlynhair, I think we went back and forth there - can you decide on the interface, so that we can align it?

[LukasWallrich]

In the CiteSource discussion, our tendency is that it should be a list, so that changing manual = from TRUE to FALSE does not break subsequent code.