Hi,
I have an issue with my contacts.
All of my contacts (except these created by facebook) were cloned. Plus the link to conversations and recent calls is broken. Now in most of my conversations and recent calls there is only the cell phone number displayed, not the picture and the name of the contact.
Also all cloned contacts were uploaded to my exchange account, so these contacts are now cloned, too.
Do you have any ideas to fix that? I think, merging the contacts would not fix the broken link issue...

I had this when first syncing contacts to Google via Nuevasync and MfE.

Your only salvation is your backup (what do you mean you didn't make a backup? ;>)

Delete your exchange settings. Upload all your optimised and checked contacts to your exchange client (Google, in my case)
Make another backup of your contacts then delete all contacts on the device. Re-establish your snc settings.

Your N900 should now download all contacts on the first sync. Subsequent syncs will tend to produce duplicates; these may be whole duplicates or duplicate fields within some contacts.

Google has an engine to remove duplicates and allows you to edit contacts to ensure they have the same format.

Ongoing syncs will produce sporadic duplicates and you'll need to check periodically, but sooner or later you should see your contacts stabilise.

Tips:

Keep contacts simple. Multiple fields on the same contact seem to increase the chance of duplicates; consider splitting some details off to make a new contact with a similar name: e.g. 'John - Work' and 'John - Home'

Initially, make changes at the client end (if possible)

Only sync one device to one client; if you run multiple devices sync one initially, then add the second. before you add a secon device, make sure it has no contacts beforehand.

Keep backups - and good luck, as that seems to be part of the process!

OR search for a plugin called "Merge your Duplicate contacts". It will then be available via drop down menu in contacts app. It does what it says. It will detect your duplicate contacts and where there are conflicts it will ask you to resolve them.

I didn't mention "Merge your Duplicate contacts" because I found it actually caused more duplicates when used in conjunction with Google, MfE and Nuevasync, but it is a very useful tool on the N900.

I use "Merge..." to double-check for duplicates but do all my editing & merging on Google's webapp and it seems to work better - I am currently not getting and duplicate contacts and only the occasional field duplicated within any of my 500-odd contacts syncing between my N900 and N8.

Thank you for your answers!

I merged the contacts with "Merge your duplicate contacts" which I use for quite a while.
And I hadn't bet on it but this also fixed the issue with the unlinked coversations and calls.