"I have been making a duplicate of the catalogue of my father's books and pictures." (5)

I have never really noticed the word duplicate before which means there is at least another copy already in existence.

My first thought was that it was for any eventual accountability of any renters, now and in the future but if another/other catalogue/s are in existence there must be some further reason.

Since portraits and books are an important part of their heritage and they were part of Anne's everyday life, having a catalogue of them with her might be comforting for her.