Checking for dupes on Compact Insert Statement
Why insert into tblusers directly at all? 

I always use staging tables. You can use SSIS of course for the same result at with greater complexity

INSERT INTO [staging].[Users]
       ([username], [password]) 
VALUES ('user1', 'pass1'), 
       ('user2', 'pass2')

INSERT INTO [tblUsers]
       ([username], [password]) 
SELECT DISTINCT [username], [password] --edit, added DISTINCT 
FROM [staging].[Users] SU
WHERE NOT EXISTS (SELECT * FROM [tblUsers] U WHERE ...)