code: function getDuplicatedChar(operandStr1, operandStr2) {
      var i = 0;
      var len = operandStr1.length;
      var pool = {};
      var dupl, key;
  
      for (; i < len; i += 1) {
          key = operandStr1.charAt(i);
          pool[key] = 1;
      }
  
      for (i = 0, len = operandStr2.length; i < len; i += 1) {
          key = operandStr2.charAt(i);
          if (pool[key]) {
              pool[key] += 1;
          }
      }
  
      pool = collection.filter(pool, function(item) {
          return item > 1;
      });
  
      pool = object.keys(pool).sort();
      dupl = pool.join('');
  
      return dupl;
  }
docs: Return duplicate charters
  @param {string} operandStr1 The operand string
  @param {string} operandStr2 The operand string
  @private
  @memberof tui.util
  @returns {string}
  @example
  //-- #1. Get Module --//
  var util = require('tui-code-snippet'); // node, commonjs
  var util = tui.util; // distribution file
  
  //-- #2. Use property --//
  util.getDuplicatedChar('fe dev', 'nhn entertainment'); // 'e'
  util.getDuplicatedChar('fdsa', 'asdf'); // 'asdf'