code: func checkDuplicateColumnName(indexColNames []*ast.IndexColName) error {
  	for i := 0; i < len(indexColNames); i++ {
  		name1 := indexColNames[i].Column.Name
  		for j := i + 1; j < len(indexColNames); j++ {
  			name2 := indexColNames[j].Column.Name
  			if name1.L == name2.L {
  				return infoschema.ErrColumnExists.GenByArgs(name2)
  			}
  		}
  	}
  	return nil
  }
docs: // checkDuplicateColumnName checks if index exists duplicated columns.