I was using Thunderbird in Windows 7. I had a separate local folder, called Email Archive, where I stored older email messages. I wanted to eliminate duplicate messages from that folder. This post briefly presents the steps I took.
I think I was using Thunderbird 11. I had previously tried using an add-on called Remove Duplicate Messages 0.1.13. I thought I was using it correctly. Whether I was or not, it deleted emails that were not duplicates. I decided to try using another add-on that was apparently a rewrite of that older program. This other add-on was called simply Remove Duplicate Messages (Alternate) (RDMA).
Unfortunately, RDMA would not install on my existing version of Thunderbird. I decided to upgrade. As it turned out, however, the current version, Thunderbird 17, was having problems. Specifically, when I would open Thunderbird, it would open twice. There were two windows, two sessions. If I tried to kill one, both would die. There didn’t seem to be a fix. It was time to bail out. I later found a webpage discussing reversion to an older version of Thunderbird, but I didn’t use its steps. I just went to Windows > Control Panel > Programs and Features. There, I uninstalled Mozilla Thunderbird.
For some reason, uninstalling did not completely uninstall. In this case, that was great. I tried again, installing Thunderbird version 15 rather than 17. I just went with the default installation options. Somehow, that was good enough: Thunderbird was back up and running, looking identical to what I’d had in version 11, before I took that version 17 detour. I may have been helped, in this, by an earlier decision to save my Thunderbird profile (which, I guess, included my email folders) on drive D, rather than on drive C. I did that because I was in the habit of backing up D, my data drive, on a daily basis; not so drive C. I don’t really know how Thunderbird, being installed to drive C, managed to figure out that it was still supposed to look on drive D for its data. But it did. So, you know, hallelujah.
So now I had an updated Thunderbird. I tried again with RDMA, and this time it installed. The installation process involved going to Thunderbird > Tools > Add-ons. Thunderbird now had two tabs open: one was the regular interface, where I could click on various email folders, and the other was this tab labeled Add-ons Manager, where I could see that I already had a couple of add-ons installed. In the Add-ons Manager tab, I clicked on the gear icon at top center and chose Install add-on from file. I had downloaded the .xpi file containing the RDMA add-on, and I would keep it for future re-installations. So now I just had to navigate to that .xpi file, using the window that opened when I clicked the Install Add-On option, and proceed with the installation from there.
Now I could see, in the Add-ons Manager tab, that RDMA was installed. It was time to configure it. To do that, I clicked on its Options button. In the General Settings tab, I had both of the top two boxes selected; and in the Message Comparison tab, I started out by selecting every box in the list of Message Elements Used as Comparison Criteria, with Seconds as my time comparison resolution. This was intended to identify only those email messages that were exact duplicates. In this first pass, I wanted to take a look at what the add-on was doing, and satisfy myself that it was not deleting things it should not have deleted.
RDMA was designed to work with a specific folder. In this case, as noted above, I wanted to use it on my Email Archive folder. I went to the other Thunderbird tab, the one showing my email folders, and I right-clicked on the Email Archive folder. That gave me an option to Remove Duplicates. I chose that option. After a moment, a new window opened. It showed me the duplicates it had found. At the bottom was an indication of how many emails it found and how many marked for deletion. The buttons at the top right allowed me to choose which messages would be deleted, out of a set of two (or more) duplicates. Clicking on these changed the indications at the bottom. It was also possible to select individual emails for deletion. This part did not display properly onscreen, but it did seem to change the indications at the bottom correctly. If in doubt, I had to unselect everything and try again before clicking OK. Note that clicking on an individual message would also display that particular message in the main Thunderbird window, so I could manually compare emails that RDMA had identified as duplicates.
When I clicked OK, RDMA deleted duplicates. Then I went back to the Add-ons Manager tab and changed the RDMA options. I did this one option at a time. Now that I had deleted a boatload of duplicates, these step-by-step comparisons found few additional duplicates. For example, unchecking the “Number of lines in message” option, and then running the comparison on the Email Archive folder, gave me only a few additional duplicates. I was thus able to do manual comparisons for these non-identical pairs. An example of a non-identical duplicate would be an email message that, for some reason, got saved in two different forms, where one had an extra blank line at the end. They really were duplicates, but they were not exactly identical.
It took some patience, and some tinkering, to figure out what difference the various message comparison options would make. In the end, I was able to work through various combinations until there would be no duplicates (or at least none that I hadn’t already checked out) even when emails were being compared on only one or two criteria. It would have been crazy to start in that way, with hardly any comparison criteria being checked; there would have been no way of knowing what criteria mattered. But by starting in with the most conservative approach and then gradually backing off, I wound up with what appeared to be an elimination of duplicates that did not inadvertently eliminate genuinely distinct messages.