k if the crash db already knows about the crash signature.

        Check if the report has a DuplicateSignature, crash_signature(), or
        StacktraceAddressSignature, and ask the database whether the problem is
        already known. If so, return an URL where the user can check the status
        or subscribe (if available), or just return True if the report is known
        but there is no public URL. In that case the report will not be
        uploaded (i. e. upload() will not be called).

        Return None if the report does not have any signature or the crash
        database does not support checking for duplicates on the client side.

        The default implementation uses a text file format generated by
        duplicate_db_publish() at an URL specified by the "dupdb_url" option.
        Subclasses are free to override this with a custom implementation, such
        as a real database lookup.
        Z