Closed "Codex Already Known" for unknown codex entries

I'm getting a lot of random codex books that I can't use to gain new entries - instead, they pop the notice "Codex already known" at the bottom of the screen, despite my having never collected that codex before.

This seems to become an issue after playing in multiplayer with other people using a character. I'd initially thought it might be because I know the codex in multiplayer but not in singleplayer, but this also seems to trigger on codex entries I haven't found in either.

Replication for me is as simple as creating a character, playing in multiplayer and learning [any] codex, and then attempting to learn codexes in singleplayer. Though a few times I think I've gotten this in singleplayer with characters I hadn't used in multiplayer....