The seen/unseen status in my library change whenever one of my friends watch a film/an episode

Are you using the trakt.tv plugin?
If so, you must create a filter in its preferences.
https://github.com/trakt/Plex-Trakt-Scrobbler/wiki/FAQ---Scrobbling#2-why-are-videos-watched-by-other-users-being-scrobbled-to-my-profile

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