What does the duplicate error mean when trying to add entities in EF Core?

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The duplicate error in Entity Framework Core (EF Core) occurs when attempting to add an entity with the same primary key as an existing entity in the database. This means that the database already has a record with the same key, and attempting to insert another record with the same key violates the unique constraint of the database. It is important to ensure that the primary key values of new entities are unique before inserting them into the database.