I have no experience with this. PresentationBuilder, which was the most ambitious project I have taken on with PresentationML, works at the granularity of a slide. This is, I think, the code you are referring to when noting that I keep a list of images. This was implemented because many presentations reuse the same image on many slides, and without deduplicating images, the presentation gets too big.
But I have never looked into de-duplicating other objects.
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