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The check runs where entities are added, which is the server side — install it on the server (or on the host in single-player) and it covers everyone. Clients need nothing.
It is safe to add to an existing world: it never touches entities that are already fine, and it stores no data of its own. Removing it puts the original behaviour back, though any identifiers it repaired stay repaired, which is exactly what you want.
Because it only acts on a clash, a world with no duplicates behaves as if the mod were not installed.
