Deleting duplicate record in SQL Server

You need to reference the CTE in the delete statement...

WITH a as
(
SELECT Firstname,ROW_NUMBER() OVER(PARTITION by Firstname, empID ORDER BY Firstname) 
AS duplicateRecCount
FROM dbo.tblEmployee
)
--Now Delete Duplicate Records
DELETE FROM a
WHERE duplicateRecCount > 1

 DELETE duplicates FROM
(SELECT ROW_NUMBER() OVER (PARTITION BY firstname, lastname, EMPNO, salary,dept  ORDER BY empno) cnt
 FROM tblEmp)  duplicates
WHERE duplicates.Cnt > 1