When I was evaluating eM Client a couple of months ago, one of the things that grabbed my attention was the “Deduplicator…” option in the Tools menu.
Making a long story short, a few years ago I got into a situation with Postbox where trying to move messages around ended up duplicating them in huge numbers. The software was not providing user feedback so I didn’t know at the time what was happening. I later learned the process would first make a copy before an intended deletion at the original location, but the server kept running into issues at different points and the process would be interrupted. So I didn’t know what was happening until I looked into it manually… After several failed attempts and trying different methods, by the end when I investigated, I found I had approximately 10x as many messages as I should! In total, I had more than 250,000 duplicates… 
Next, I wanted to remove these. Postbox struggled with trying to view all the messages and removing them was not viable. I spoke to the Postbox team to look for a solution, but there was not one. I could not find a duplicate removal tool in Outlook (Mac version), although I’d read about such a feature in a Windows version. I looked at third party tools, but did not trust them sufficiently. I tried the webmail version of Outlook and deleting a few messages at a time, but I would have to scroll to get it to load the next few messages each time and this just wasn’t viable either. Finally, I talked to the IT team and they didn’t have a solution either, but suggested I could work my way through it manually.
One folder, for one particular year, alone had 195,000 duplicates. Not surprisingly, it was going to be hard to tackle this! During the weekend, I decided to try out eM Client’s Deduplicator feature. It came up with a processing window which I thought I’d need to leave alone for hours or days, but very quickly afterwards it displayed a report about the number of duplicates. I was a bit shocked. I told it to go ahead and how to remove the duplicates (move to Trash) and then left it for hours.
I was quite shocked and extremely grateful when I found this had actually worked and had dealt with a problem I’d not been able to address for several years! 
It also meant I could free up a lot of my quota again.
I’ve found a few more apparent duplicates that I didn’t see in Outlook via webmail, but closer inspection revealed these were other copies in other folders I’d not addressed yet (such as Drafts, Sent mail, or the Archive) and so eM Client was actually correctly revealing linked messages across conversations/threads that Outlook was not showing me.
This feature has been a tremendous help and I was shocked by what it was able to handle. A very big thank you to the developers! 