Somewhere you have two files that state the class as flat2 ...

Usually this happens when a back up file is left on the server or when you have accidentally copied the code file into the language file ...

Do a search in the Tools ... Developers Tool Kit ... for:
flat2

Where are these files and do you have two files in the:
/includes/modules/shipping
-- or --
/includes/languages/english/modules/shipping

that have flat2 in them?