Merge Customer Records with same Email address that is not being detected as Duplicate Records

Solution

Enable Duplicate Detection for Customers and Vendors with different Subsidiaries

1. Navigate to Setup > Company > Duplicate Detection 
2. Detect Duplicates Across Subsidiaries (Customers and Vendors Only):Enter Checkmark
3. Click Save

Merge the Customer records

1. Navigate to Lists > Relationship > Customers
2. Click Edit to the Customer you would like to set as a Child record
3. Select Master Customer Record
   Note: This is where the child record will be merged.
4. Click Merge
5. Click OK
6. Click Refresh