Seems you have not correctly written your SP, try this way.
ALTER PROCEDURE [dbo].[avoidduplicate]
@username varchar(20),
@pword nvarchar(20),
@empname varchar(20),
@email nvarchar(50),
@designation varchar(20),
@reportto varchar(20),
@Result varchar(50) output
as
BEGIN
if exists(Select username from testduplicate where username = @Username)
begin
set @Result = 'Already there'
return
end
insert into testduplicate(username,pword,empname,email,designation,reportto)values(@username,@pword,@empname,@email,@designation, @reportto )
set @Result= 'Success'
END