Dialer & Contacts app display duplicate (WhatsApp)
I solved this issue by editing all my Google contacts to make sure they contain full country code and no dashes or spaces between the numbers. In the Google Contacts landing page (web) they will all look OK but once you click edit you might find some with the incorrect format. Doing this will enable the Dialer app to link the mobile and Whats App contacts correctly. I know this is still a tedious manual fix but it is the only one I found to work.