Speaking as the operator of u/[email protected] … I think it’s kind of inevitable with the current design of lemmy for folks that browse “all” instead of subscribed … and in some cases local.
As an example, Auto_Post_Bot posts news post to !zed and !zed (the latter is recent per request from the admin over there, as they don’t want centralized communities)… So if your instance is “subscribed” to both, it’s going to be a “duplicate” post in the “all” feed.