We had this one customer who emailed about a broken checkout, waited maybe twenty minutes for a reply that hadn't come yet, then emailed again from his phone, then somehow also submitted our web form on top of that. Three tickets, three agents, and each one was drafting a response with no clue the other two existed. Now the related tickets panel actually flags likely duplicates from the same person, so whoever opens the ticket sees a little red badge and can merge before wasting time writing something the customer's about to get twice more anyway. Review collected by and hosted on G2.com.
It only catches tickets from the same requester, so if someone emails from a personal address then later from their work one about the exact same issue, nothing links them and I've still had to catch a couple of those manually just by noticing the subject line looked familiar. Review collected by and hosted on G2.com.