'Logical file name "MyDB" is already in use' Even though the database doesn't exist
If i am able to figure out correctly from below query 

ALTER DATABASE [tempdb] MODIFY FILE ( NAME = N'DDAS_log', FILENAME = N'G:\DDAS_log.ldf')


I believe tempdb to be [DDAS] here..

Also, in the CREATE DATABASE Query

IF NOT EXISTS (SELECT * FROM sys.databases WHERE name = 'DDAS')
BEGIN
    CREATE DATABASE [DDAS]
    CONTAINMENT = NONE
    ON  PRIMARY 
        ( NAME = N'DDAS', FILENAME = N'E:\DDAS.mdf' , SIZE = 1024000KB , MAXSIZE = UNLIMITED, FILEGROWTH = 1024000KB  ) 
    LOG ON      
        ( NAME = N'DDAS', FILENAME = N'G:\DDAS_log.ldf' , SIZE = 1024KB , MAXSIZE = 2048GB , FILEGROWTH = 10%)
    END
GO


Above the logical file name of log file seems wrong, instead of LOG ON
            ( NAME = N'DDAS'....) it should have been NAME = N'DDAS_log'..

For more info please refer CREATE DATABASE