Double contacts after syncing with Outlook
I fixed this by manually setting the "middle name" to "A" on all Outlook contacts, then synced with the iPhone, then manually deleted all contacts without an "A" middle name on the iPhone, then removed the "A" middle names in Outlook and then synced with the iPhone again.

Drawbacks: lots of manual work and loosing the middle name content (luckily not used in my contacts).

Using the middle name is good because it is shown in list view on the iPhone, so one can quickly see what must be removed without looking at details.