What issue do you think that Thunderbird should fix? Automatically saving addresses? That is a feature that some users wish to use. For others it is not necessary or desirable but it is up to the user to determine that and switch off the functionality.
What issue do you think that Thunderbird should fix?
duplicating issue not auto saving address. For me "auto saving address" is very important option but with duplicated contacts it make TB as hell, 12000 contacts with 50% duplicates (I suspect) not desirable percent for any user.
In how many address books are these contacts stored? Are you moving contacts from one address book where they are collected by Thunderbird to another address book for use?
Then there should be no duplication as a result of Thunderbird collecting addresses from outgoing messages. Are you selecting addresses to save from incoming messages, or adding (manually or importing) contact data from other sources?
Then you should review your procedures. Thunderbird won't identify duplicate records in imported data so the elimination of duplicates should be done before importing or the address data must be reviewed and duplicates removed after the import.
The manual trick shared in the above blog post doesn't require any 3rd party tool or any cost. All you need to use is MS Outlook client, that's it. It can help you remove both duplicate emails and duplicate contacts from PST files.
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