Console magic

You can detect duplicate ids by evaluating this Javascript expresssion in Chrome devtools console:

([].slice.call(document.querySelectorAll("[id]")))
    .map(function(ele){ return ele.id;})
    .sort()
    .reduce(function(countMap, word) {countMap[word] = ++countMap[word] || 1;return countMap}, {})

Boom! You will get:

> Object { 
   id1: 1, 

   id2: 1,
   some-other-id: 1,
   duplicate-id: 2
   …
}

What is going on here:

• document.querySelectorAll() returns the NodeList of elements that have id attribute
• slice() - converts a NodeList to a JavaScript Array
• map() - converts elements array to array of id strings
• sort() - sorts the array. If you evaluate only up to here you will get the sorted list of ids (with duplicates)
• reduce() - creates an object with id value as key and number of occurrences as value

Additionally, if you add Object.keys(resultFromAbove).sort() you will get a sorted list of unique ids.

Also an example of Map-Reduce 

BTW you can use similar, slightly modified selector expression to list the CSS classes that are used on the page and also mow many times.