You have placed a constraint (either UNIQUE, PRIMARY KEY, FOREIGN KEY, or CHECK) on a set of columns, but a constraint of the same type on those columns already exists. In the case of a FOREIGN KEY, this error only occurs if the same foreign-key set of columns references the same existing parent key set of columns. Either the constraint was established when the table was created, or it was added later. Because the constraint already exists, it is not added again.