To prevent Case submitted by a Contact from being associated to Anonymous Customer when contact is associated to a Company with multiple Contacts, perform the following steps:
1. As Administrator navigate to Setup > Company > Duplicate Detection.
2. On the Set Up Duplicates Detection Feature window, set the following:
a. Check Detect Customer Duplicates.
b. Fields to Match On = Name/ID.
c. Check Detect Contact Duplicates.
d. Fields to Match On = Company/Email (full).
3. Navigate to Setup > Support > Online Case Forms.
4. Edit the Online Case Form.
5. In the Select Fields tab, locate the Email field and check the Search field.
6. In the Set Up Workflow tab, uncheck Use Duplicate Detection Criteria.
7. Click Save.
8. In the External tab, click Publishable Form URL.
9. Fill all mandatory fields.
10. Click Submit.
11. Navigate to Lists > Support > Cases.
12. Open the case to verify that is it associated to the correct Customer.