You must scour every field in both the Contact and Account objects for a possible suspect.
To do this:
- Click your name at the top.
- Select Setup.
- On the left side under App Setup, select Customize > Accounts *[Contacts] > Fields.
- Scroll down to the custom fields.
- Examine the custom fields for any with the data type of Contacts(Lookup) *[Accounts(Lookup)].
- When you find one, cross-examine with the standard and custom fields that exist and identify a potential duplicate by comparing the internal field names.
Account Custom Field:
Public Name: Org Proxy Contact
API Name: Contact__c
Data Type: Lookup
The issue is that this account custom field is having to perform a lookup of a contact field with the name of Contact__c. However, this directly conflicts with a field that is labeled as cv__Contact__c (Public Name is Contact). While they look different, Salesforce has trouble recognizing the difference, because what is inside the underscores is the same.
*If you do not find a field in the Accounts object, then switch over and peruse the Contacts object.
Part B: Adjusting the custom field
After identifying the field, change the internal name to something else, something more customized that would not already exist, and then merge again.
To do this:
- Click your name at the top.
- Select Setup.
- On the left side under App Setup, select Customize > Accounts *[Contacts] > Fields.
- Scroll down to the custom fields and select "Edit" next to the custom field.
- Change the internal name, or the API name, to something different.
- Save.
- Merge the duplicates again.