Duplicate or incomplete contacts are a rare issue on the iPhone, and are almost always caused by old contact data.
To resolve this issue:
1. Delete your YouMail contacts
2. Reupload your contacts.
As a last resort, you can turn off the YouMail contacts sync from your address book. This measure will prevent YouMail from adding contacts to your phone's address book but it will also not load your contacts in the YouMail app.
If you are still having problems, please submit a ticket to our support team.