code: func CheckOnce(cols []*Column) error {
  	m := map[string]struct{}{}
  	for _, col := range cols {
  		name := col.Name
  		_, ok := m[name.L]
  		if ok {
  			return errDuplicateColumn.Gen("column specified twice - %s", name)
  		}
  
  		m[name.L] = struct{}{}
  	}
  
  	return nil
  }
docs: // CheckOnce checks if there are duplicated column names in cols.