declare 
  column_exists exception;
  pragma exception_init (column_exists , -01430);
begin 
  execute immediate 'ALTER TABLE FISCAL_ML2 MODIFY (DESCTIPODOC VARCHAR2(18))';
  exception when column_exists then null;
end;
/
declare 
  column_exists exception;
  pragma exception_init (column_exists , -01430);
begin 
  execute immediate 'ALTER TABLE FISCAL_ML2 MODIFY (DESCTIPODOC2 VARCHAR2(18))';
  exception when column_exists then null;
end;