code: function reportDuplicateRules( ast, session ) {
  
      const rules = {};
  
      const check = session.buildVisitor( {
          rule( node ) {
  
              const name = node.name;
  
              if ( __hasOwnProperty.call( rules, name ) ) {
  
                  const start = rules[ name ].start;
  
                  session.error(
                      `Rule "${ name }" is already defined at line ${ start.line }, column ${ start.column }.`,
                      node.location
                  );
  
              }
  
              rules[ node.name ] = node.location;
  
          }
      } );
  
      check( ast );
  
  }
docs: Checks that each rule is defined only once.