How to insert data in sql server table using user defined table types, and check duplicate data?

I am using C# to upload excel file data in sql server. I am creating a datatable and passing it to SP. I've created SP like below

    Create PROCEDURE [dbo].[usp_InsertData]

  @dt AS dbo.tbl_employees READONLY,
  @CREATEDBY as varchar(50),
  @folderPath as nvarchar(3000),
  @result as varchar(100) OUTPUT

AS
BEGIN
 

 INSERT INTO Employees(Name, Email, Branch, CreatedBy, FolderPath)
  SELECT Name, Email, Branch,@CREATEDBY, @folderPath
  FROM @dt;	
set @result='Success';


select @result
Print @result
END	

But it's not giving me output value. Also I want to validate duplicate data from user-defined-table(dbo.tbl_employees) with actual table (Employees). If there are any duplicate data then it should give a message 'Duplicate data'. Don't know how to do it. Can someone help me to make it work?
Thanks

You should have a unique constraint on your table to ensure you don't get duplicate data.

But if you want a special return value (rather than just throwing an error), you need to check your data first, and conditionally return a value.

CREATE OR ALTER PROCEDURE dbo.usp_InsertData
  @dt AS dbo.tbl_employees READONLY,
  @CREATEDBY as varchar(50),
  @folderPath as nvarchar(3000),
  @result as varchar(100) OUTPUT
AS

SET XACT_ABORT ON;

BEGIN TRAN;

IF EXISTS (SELECT 1
    FROM Employees e WITH (HOLDLOCK)
    JOIN @dt dt ON dt.Name = e.Name
)
BEGIN
    SET @result = 'Duplicate data';
    ROLLBACK;
    RETURN;
END;

INSERT INTO Employees
    (Name, Email, Branch, CreatedBy, FolderPath)
SELECT Name, Email, Branch, @CREATEDBY, @folderPath
FROM @dt;

SET @result = 'Success';

COMMIT;

If you are having problems getting information out of an OUTPUT parameter in C# then that is a separate question. There are already many posts that show you how to do that.