Cannot insert duplicate key in object 'dbo.Users'. The duplicate key value is (<NULL>) when id auto increment code example

Example 1: Cannot insert duplicate key in object 'dbo.Instructor'. The duplicate key value is (2).

DBCC CHECKIDENT ('TABLE_NAME_GOES_HERE', RESEED, ZZZZZ);

Example 2: Cannot insert duplicate key in object 'dbo.Instructor'. The duplicate key value is (2).

DBCC CHECKIDENT ('TABLE_NAME_GOES_HERE', NORESEED);