detect-reddit-comment-copy.user.js

// ==UserScript==
// @name         Detect Reddit Copy Bot Comments
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Detect copied comments in Old Reddit.
// @author       qgustavor
// @match        https://*.reddit.com/*
// @icon         https://icons.duckduckgo.com/ip2/reddit.com.ico
// @grant        none
// ==/UserScript==

(function() {
  'use strict';
  const comments = Array.from(document.querySelectorAll('.comment:not(.deleted) > .entry > .usertext > .usertext-body')).map(e => {
    const text = e.textContent.trim() || e.innerHTML
    const timestampEl = e.parentNode.parentNode.querySelector('.live-timestamp')
    const timestamp = timestampEl && new Date(timestampEl.getAttribute('datetime')).getTime()
    return [text, timestamp, e]
  }).filter(e => e[1]).sort((a, b) => a[1] - b[1])

  const duplicatedComments = comments.filter((needle, i) => (
    comments.some(e => needle[1] > e[1] && e[0] === needle[0]) ||
    (needle[0].length > 20 && comments.some(e => needle[1] > e[1] && e[0].includes(needle[0])))
  ))
  duplicatedComments.forEach(e => {
    e[2].style.background = 'red'
  })
})()

You should know that it may show false positives on popular non-creative comments (e.g. "Holy hell!").