We plan to provide such functionality in the future.
In the meantime: If you are using a Mac, the Addressbook application has a built-in deduplication function that could help here.Peter Mueller commented
A de-duplication feature was promised some 5 years ago, but still is not in place.
BTW, instead of merging duplicated entries, it would be more efficient to simply ignore already existing entries when syncing. Should not be hard to implement.Peter Mueller supported this idea ·Peter Mueller commented
Somehow fruux and the Thunderbird and/or Android sync tools managed to produce thousands of duplicates in my address book with some 1200 entries. The database blew up to over 14.000 entries (which BTW makes the web interface showing them extremely slow!)
Since I found no way to get rid of the duplicates (or the whole address book) I resorted to deleting my account and setting up a new one. When I then synced my TB address book for the first time, all local entries disappeared! Thanks goodness I was prudent enough to make a backup before. When I then restored the entries by copying them in and then synced, the online version had all entries duplicated. When I synced again the duplicates also showed up on TB. By applying Duplicate Contacts Manager: https://addons.thunderbird.net/en-US/thunderbird/addon/duplicate-contacts-manager/ extension, I was able to get rid of the local duplicates. Yet in the online fruux database they are not deleted by (manual) synchronization. WTF.