Double contacts after syncing with Outlook

I have duplicate contacts in my iPhone, not knowing their source or how to safely delete them.

I synced my iPhone contacts with Outlook via iTunes. I have 220 contacts in Outlook. I estimate 100 of these are duplicates in the iPhone i.e in both the Contact and Phone app. They are apparently exact duplicates.

The strange thing is that the duplicate ones do not get synced to Outlook so there are no duplicate contacts there.

I don't know which of two contacts is linked to the respective Outlook contact, so I fear that simply deleting one contact may delete it in Outlook as well, which would be bad.

Does anyone have any ideas on the cause and cure for this behaviour?

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.

I encountered the exact same problem but with Address Book, to the point where I had duplicate contacts both on the phone and in Address Book. To get around it, I went through Address Book, deleted all of the dupes (and they were all exact duplicates), and then synced Address Book for the iPhone with Address Book on my Mac such that it deleted all of the contacts on the phone and just started fresh. I still have no idea how it happened so I'm not sure how to prevent it from happening again beyond tying the contacts to Address Book from now on, though that's not how I like to handle things. Syncing is a hard thing to program, but I would think Apple would be better at it than this.