Find out easily what anime you have rewatched by executing this script on your myanimelist.net 'Completed' list page.

mal-rewatch-counter.js

/*
* MyAnimeList Rewatch Counter
* (+ reread manga)
*
* This is a tool to easily and quickly make a list of what anime you have rewatched and how many times.
* Also works for reread manga.
*
* HOW TO USE:
* Go to a myanimelist.net anime/manga list, and choose the 'Completed' view.
* Open a JavaScript console on your browser and paste this code into it and press enter.
* The script will read all 'More' views on the list, and after a few seconds generate the rewatch/reread list.
*
* NOTE on the new default list style:
* The script will work, but you will have to manually scroll the list to load all titles onto the page before executing.
*/

var url = window.location.href.split('?')[0];
var type = "";

if (url.indexOf("myanimelist.net/animelist") > -1) {
  type = "anime";
} else if (url.indexOf("myanimelist.net/mangalist") > -1) {
  type = "manga";
} else {
  alert("Execute this on a MyAnimeList.net anime/manga list page!");
  throw new Error("Execute this on a MyAnimeList.net anime/manga list page!");
}

// Works only on the 'Completed' page
if (getParameterByName('status') != 2) {
  alert("Execute this on the 'Completed' page! \nRedirecting. \nTry again after the page loads.");
  window.location.replace(url + "?status=2");
  throw new Error("Redirecting");
}

var username = url.split('/').pop();

var titles_old = document.querySelectorAll('div table tbody tr a.animetitle span');
var titles_new = document.querySelectorAll('tbody.list-item tr.list-table-data td.data.title a.link.sort');
var titles = [];
var old_list = false;

if (titles_old.length > titles_new.length) {
  titles = titles_old;
  old_list = true;
} else {
  for (var i = 0; i < titles_new.length; i++) {
    titles[i] = titles_new[i].innerHTML;
  }
}

var rewatches = [];
var moreLinks = document.querySelectorAll('a');
var resultArray = [];
var result = 'data:text/html;charset=utf-8,<style>html,body{margin: 0;padding: 0;}</style><div style="white-space: pre;width: 100%;max-width:650px;font-size: 18px;color:#444;height: 100%;border: none;outline: none;margin:40px auto;padding: 30px;">';

if (type == "anime") result += "<h1>Rewatched anime</h1>";
if (type == "manga") result += "<h1>Reread manga</h1>";

if (type == "anime") result += "<h3><em>How many times " + username + " has seen it</em></h3><br>";
if (type == "manga") result += "<h3><em>How many times " + username + " has read it</em></h3><br>";

if (old_list) {
  // Fetch rewatch count information bypassing the 'More' link
  $("div.hide").each(function(index,value){
    var series_id = $(value).attr('id').split('more')[1];
    $.post("/includes/ajax-no-auth.inc.php?t=6", {color:1,id:series_id,memId:$('#listUserId').val(),type:$('#listType').val()}, function(data) {
      if (type == "anime") rewatches[index] = $(data.html).find('strong')[0].innerHTML;
      if (type == "manga") {
        var moreSection = $(data.html).find('td').html();
        var timesReadIndex = moreSection.indexOf("Times Read");
        rewatches[index] = moreSection.charAt(timesReadIndex + 12);
      }
    }, "json");
  });
} else {
  // Click all links labeled 'More' to get the rewatch counts on page
  for( var i=moreLinks.length; i--; ) {
    if (moreLinks[i].innerHTML == 'More') {
      moreLinks[i].click();
    }
  }
}

// Progress bar
$('body').append('<div id="progress" style="width: 600px;height: 20px;position: fixed;top:30%;left:50%;margin-left:-300px;border: 5px solid #ccc;"></div>')
    .append($('<div id="bar" style="text-align: left;width: 0px;height: 20px;position: fixed;top:30%;left:50%;margin-left:-295px;margin-top:5px;background-color: #479af9;white-space:pre;color:white;font-weight:bold;font-size:16px"></div>'));

// Repeat every 1 second until all More-sections are processed
wait();

function wait() {
  setTimeout(function () {
    if (!old_list) rewatches = document.querySelectorAll('tbody.list-item tr.more-info strong');

    // Log progress
    console.log(rewatches.length + "/" + titles.length + " opened");
    $('#bar').text(" " + rewatches.length + "/" + titles.length);
    $('#bar').css('width', rewatches.length / titles.length * 600 + 'px');

    if (rewatches.length != titles.length) {
      // All sections aren't open yet, check again after 1 second
      wait();
    } else {
      if (old_list) {
        // OLD list:
        for (var i = 0; i < titles.length; i++) {
          // Parse rewatched shows into an array of arrays with rewatch count as index
          if (rewatches[i] > 0) {
            if (resultArray[rewatches[i]]) {
              resultArray[rewatches[i]] = resultArray[rewatches[i]].concat("<li>" + titles[i].innerHTML + "</li>");
            } else {
              resultArray[rewatches[i]] = "<b>" + (parseInt(rewatches[i]) + 1) + " times:</b>"; // +1 to change from rewatch count to watch count
              resultArray[rewatches[i]] = resultArray[rewatches[i]].concat("<ul>");
              resultArray[rewatches[i]] = resultArray[rewatches[i]].concat("<li>" + titles[i].innerHTML + "</li>");
            }
          }
        }
      } else {
        // NEW list:
        for (var i = 0; i < titles.length; i++) {
          // Parse rewatched shows into an array of arrays with rewatch count as index
          if (rewatches[i].innerHTML > 0) {
            if (resultArray[rewatches[i].innerHTML]) {
              resultArray[rewatches[i].innerHTML] = resultArray[rewatches[i].innerHTML].concat("<li>" + titles[i].trim() + "</li>");
            } else {
              resultArray[rewatches[i].innerHTML] = "<b>" + (parseInt(rewatches[i].innerHTML) + 1) + " times:</b>"; // +1 to change from rewatch count to watch count
              resultArray[rewatches[i].innerHTML] = resultArray[rewatches[i].innerHTML].concat("<ul>");
              resultArray[rewatches[i].innerHTML] = resultArray[rewatches[i].innerHTML].concat("<li>" + titles[i].trim() + "</li>");
            }
          }
        }
      }

      resultArray.reverse();

      resultArray.forEach(function (value, index, array) {
        result += value.concat("</ul>");
      });

      result += "<br><br><br><br></div>";
      window.location = result;
    }
  }, 1000);
}

function getParameterByName(name) {
  name = name.replace(/[\[]/, "\\[").replace(/[\]]/, "\\]");
  var regex = new RegExp("[\\?&]" + name + "=([^&#]*)"),
      results = regex.exec(location.search);
  return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
}

-- Here is an updated version thar worked for me

var url = window.location.href.split('?')[0];
var type = "";

if (url.indexOf("myanimelist.net/animelist") > -1) {
type = "anime";
} else if (url.indexOf("myanimelist.net/mangalist") > -1) {
type = "manga";
} else {
alert("Execute this on a MyAnimeList.net anime/manga list page!");
throw new Error("Execute this on a MyAnimeList.net anime/manga list page!");
}

// Works only on the 'Completed' page
if (getParameterByName('status') != 2) {
alert("Execute this on the 'Completed' page! \nRedirecting. \nTry again after the page loads.");
window.location.replace(url + "?status=2");
throw new Error("Redirecting");
}

var username = url.split('/').pop();

var titles_old = document.querySelectorAll('div table tbody tr a.animetitle span');
var titles_new = document.querySelectorAll('tbody.list-item tr.list-table-data td.data.title a.link.sort');
var titles = [];
var old_list = false;

if (titles_old.length > titles_new.length) {
titles = titles_old;
old_list = true;
} else {
for (var i = 0; i < titles_new.length; i++) {
titles[i] = titles_new[i].innerHTML;
}
}

var rewatches = [];
var moreLinks = document.querySelectorAll('a');
var resultArray = [];
var result = '';

if (type == "anime") result += "Rewatched anime\n";
if (type == "manga") result += "Reread manga\n";

if (type == "anime") result += "How many times " + username + " has seen it:\n\n";
if (type == "manga") result += "How many times " + username + " has read it:\n\n";

if (old_list) {
$("div.hide").each(function(index,value){
var series_id = $(value).attr('id').split('more')[1];
$.post("/includes/ajax-no-auth.inc.php?t=6", {color:1,id:series_id,memId:$('#listUserId').val(),type:$('#listType').val()}, function(data) {
if (type == "anime") rewatches[index] = $(data.html).find('strong')[0].innerHTML;
if (type == "manga") {
var moreSection = $(data.html).find('td').html();
var timesReadIndex = moreSection.indexOf("Times Read");
rewatches[index] = moreSection.charAt(timesReadIndex + 12);
}
}, "json");
});
} else {
for( var i=moreLinks.length; i--; ) {
if (moreLinks[i].innerHTML == 'More') {
moreLinks[i].click();
}
}
}

$('body').append('

')
.append($('

'));

wait();

function wait() {
setTimeout(function () {
if (!old_list) rewatches = document.querySelectorAll('tbody.list-item tr.more-info strong');

console.log(rewatches.length + "/" + titles.length + " opened");
$('#bar').text(" " + rewatches.length + "/" + titles.length);
$('#bar').css('width', rewatches.length / titles.length * 600 + 'px');

if (rewatches.length != titles.length) {
  wait();
} else {
  if (old_list) {
    for (var i = 0; i < titles.length; i++) {
      if (rewatches[i] > 0) {
        if (resultArray[rewatches[i]]) {
          resultArray[rewatches[i]] = resultArray[rewatches[i]].concat("\n - " + titles[i].innerHTML);
        } else {
          resultArray[rewatches[i]] = (parseInt(rewatches[i]) + 1) + " times:\n";
          resultArray[rewatches[i]] = resultArray[rewatches[i]].concat(" - " + titles[i].innerHTML);
        }
      }
    }
  } else {
    for (var i = 0; i < titles.length; i++) {
      if (rewatches[i].innerHTML > 0) {
        if (resultArray[rewatches[i].innerHTML]) {
          resultArray[rewatches[i].innerHTML] = resultArray[rewatches[i].innerHTML].concat("\n - " + titles[i].trim());
        } else {
          resultArray[rewatches[i].innerHTML] = (parseInt(rewatches[i].innerHTML) + 1) + " times:\n";
          resultArray[rewatches[i].innerHTML] = resultArray[rewatches[i].innerHTML].concat(" - " + titles[i].trim());
        }
      }
    }
  }

  resultArray.reverse();

  resultArray.forEach(function (value, index, array) {
    result += value.concat("\n");
  });

  alert(result);
  console.log(result);
}

}, 1000);
}

function getParameterByName(name) {
name = name.replace(/[[]/, "\[").replace(/[]]/, "\]");
var regex = new RegExp("[\?&]" + name + "=([^&#]*)"),
results = regex.exec(location.search);
return results === null ? "" : decodeURIComponent(results[1].replace(/+/g, " "));
}