code: func checkDuplicateColumnName(indexColNames []*ast.IndexColName) error {
  	colNames := make(map[string]struct{}, len(indexColNames))
  	for _, indexColName := range indexColNames {
  		name := indexColName.Column.Name
  		if _, ok := colNames[name.L]; ok {
  			return infoschema.ErrColumnExists.GenWithStackByArgs(name)
  		}
  		colNames[name.L] = struct{}{}
  	}
  	return nil
  }
docs: // checkDuplicateColumnName checks if index exists duplicated columns.