Provide an interface to merge duplicate contacts
Plaxo had a great interface to identify and then manually merge duplicate contacts. It would present the offending contacts and visually mark which entries would be present in the "merged" card. With Google Contacts sync coming (I'm suffering from many duplicate contacts as a result), it would be great to have this.

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.
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Adminfruux (CEO / Founder, fruux) commented
@Peter:
Thunderbird is (sadly) not the best CardDAV/CalDAV client out there. It would be great if there was a really awesome open source desktop client (since CardDAV/CalDAV are also open standards), but sadly there is not.On the commercial side of things: On Windows www.emclient.com is a really high quality CardDAV/CalDAV client.
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Adminfruux (CEO / Founder, fruux) commented
@Peter:
We can't produce duplicates. fruux is a "dumb filesystem" for special files (contacts, calendars and tasks). Think of us as "an USB stick for these special files). We just store whatever your client app gives us.This is likely related to the malformed data in your account (see comment in other thread on here). Your client app likely comes across a malformed contact, then fails and endlessly retries some operation.
This is not the best place for support by the way - for support issues, please email us at support at fruux dot com. We can help repairing your data so this issue won't happen.
Regarding deduplication:
This is a feature that is on our list of things that we'd like to have in the web app at some point, but we don't have immediate plans for it. UserVoice sadly only has "completed", "declined" and "planned" as status. "Planned" means "We think this is a cool idea, would be great to have that at some point".Some client apps are really good at deduplication though - e.g. Contacts.app on macOS.
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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 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.