modified.js

function throttle(func, duration) {
  let shouldWait = false
  let hasIgnoredCalls = false;

  return function (...args) {
    if (!shouldWait) {
      func.apply(this, args)
      shouldWait = true
      setTimeout(function () {
        shouldWait = false
        if (hasIgnoredCalls) {
           hasIgnoredCalls = false;
           func.apply(this, args);
        }
      }, duration)
    } else {
      hasIgnoredCalls = true;
    }
  }
}

original.js

function throttle(func, duration) {
  let shouldWait = false
  return function (...args) {
    if (!shouldWait) {
      func.apply(this, args)
      shouldWait = true
      setTimeout(function () {
        shouldWait = false
      }, duration)
    }
  }
}