As with many of our threads you very likely aided and abetted a dozen others with the same symptoms, thanks!

 

Many thanks to skyshadow and others for their helpful replies. These helped me solve a similar problem we had with our 2015 RX-350.
What I think was happening on our Lexus was that even though (I think that) iCloud wasn't syncing contacts, the phone still had iCloud contacts and these were being transferred to the Lexus.

What I did to fix was:
1. (on phone) Settings > (User Name) > iCloud
2. In "apps using icloud", turn Contacts ON
3. If necessary, tell it to merge with existing contacts (aside: we use the MS Outlook app to sync our iPhones with our Outlook contacts)
4. In "apps using icloud" turn Contacts OFF
5. When asked, tell the phone to delete the contacts (this appears to delete them from the phone, they were still in iCloud afterwards)
6. Start the Lexus -> causes contacts to update
7. Duplicate entries are now gone.

Note that I didn't have to delete the Lexus's phonebook entries, re-attach the iPhone, etc to update the Lexus entries.