JSFiddle - React, Tailwind, and code Playground

JavaScript

var str = 'Hello to all from Boston';

function returnFirstRepeatChar2(str) {
  return str.split(' ').reduce(function (p, c) {
    var hasDupes = c.toLowerCase().split('').sort().join('').match(/(.)\1+/g);
    if (hasDupes) p.push(c);
    return p;
  }, []);  
}

console.log(returnFirstRepeatChar2(str))