This can be easily achieved by using regex and String.prototype.replace().

let wordsList = [
  'hheellloo',
  'hhooww',
  'aaaarrrreee',
  'yyyyyoooouuuuu',
];

const removeDuplicateLetters = (word) => {
  return word.replace(/(.)\1+/g, (match, contents) => {
    return contents.indexOf(match) + 1 ? '' : match;
  });
}

wordsList = wordsList.map(word => removeDuplicateLetters(word));

console.log(wordsList);
// ["helo", "how", "are", "you"]

You can reduce the code to only one line.

let wordsList = [
  'hheellloo',
  'hhooww',
  'aaaarrrreee',
  'yyyyyoooouuuuu',
];

wordsList = wordsList.map(word => word.replace(/(.)\1+/g, '$1'));

console.log(wordsList);
// ["helo", "how", "are", "you"]