Set One Online Case form to Not Create a New Customer record

Use "Anonymous Customer" so that no new record will be created.
There is a change in process though, because you need to inform (via Online case form message) your existing customers to use an email address linked to the anonymous customer record.
Steps:
1. Navigate to Setup > Support > Support Preferences > External tab > Anonymous Customer Placeholder =  Anonymous customer name
2. Do a Global search of the customer record from step # 1 > Edit it > Email = existing@email.com > Save
 
3. Navigate to Setup > Support > Online case forms > Edit your form
   3.1. Message field, put a message that the customer can readily see.
          sample "Message": 
          ***IMPORTANT NOTE***
If you are an existing customer, kindly use this email address: existing@email.com
 
   3.2. Select fields tab:
          Email > Search = check the box
 
   3.3. Setup Work flow tab:
          Handle duplicate records = Update the first record created
         Use Duplicate Detection Criteria = not checked
 
   3.4. Save the changes to the Online case form
 
4. Do a test by filling up the form. Populate all necessary fields.
Email = existing@email.com
Save