I'm a long-time user of AUCTeX (thanks, maintainers, for all your work on it!), and I just started using Okular as my previewer (having found that Evince 3.18 kept crashing whenever a PDF was updated, most frustrating; nothing to do with AUCTeX though!).
I noticed that Emacs seemed only ever to start one instance of Okular, which makes it impossible to view multiple files side-by-side. I tracked this down to its use of --unique. While there may well be users who prefer this style, I suggest that it shouldn't be the default in AUCTeX, because it's not the default for other viewers.
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