check duplicate letter in word js code example

Example: find repeated words in paragrah a string javascript

const str = "big black bug bit a big black dog on his big black nose";
const findDuplicateWords = str => {
   const strArr = str.split(" ");
   const res = [];
   for(let i = 0; i < strArr.length; i++){
      if(strArr.indexOf(strArr[i]) !== strArr.lastIndexOf(strArr[i])){
         if(!res.includes(strArr[i])){
            res.push(strArr[i]);
         };
      };
   };
   return res.join(" ");
};
console.log(findDuplicateWords(str));