Well, I also faced this type of concurrency error while working with Entity Framework 6.13 using Code First. And I also struggled for hours before solving it myself. So it can help someone out there. A table was created with composite primary keys and today I changed the structure of the table and made only one primary key(with AutoIncrement) instead of composite keys and update the table through migrations with fluent-api configurations. Table got updated but the primary key was not updated with AutoIncrement and whenever I tried to add a record, it showed the concurrency error. So, as I set the autoincrement of the field and the error was gone. Hope, it helps.
