What's app notification showing notification with phone number's not by contact names even after saving the contact's. Group names is also showing by their previous names (not by new names)
Is the issue with Dual what's App or the primary?
Make sure you are on the latest software version which is .86 if not, you can check by
Go to setting => System => About => Software information.
Try clearing cache and data and reinstalling the App (Make sure to backup the data first)