After the v38.5 software release, which updates your system to Salesforce's latest API Version 60, duplicate Fields are no longer permitted in any instance. If there are fields on any object which have a duplicate API Name, you will experience Errors when trying to load the page. You will first need to change the API Name of the duplicated Field.
Navigate to Setup
Go to the Objects menu and select the Object where the Field causing the error is located. In this example the duplicate field is the Customer Phone field:
But the Field could be on the Prospect, Appointment, Lead Source, Sale, or any other i360 Object.
In the example below the Sale Status field is on the Sale object. You will not be able to Edit the Managed field beginning with "i360__" so instead you will need to Edit the API Name of the unmanaged Field, in the example below it is the Sale_Status__c field.
Keep in mind, you may be seeing the Error while trying to load the Project page but the duplicate field could be on the Project, or Project Activity object. Or you may see the error when trying to load the Quote page but the Field referenced in the Error is actually on the Appointment object.
You may need to check more than one Object to find the duplicate if the Field in the error is not one you are familiar with.
Locate the Duplicated field and change the API Name to include additional characters, meaning the field API names will no longer be the same:
Next, you'll need to run First Time Setup. If you are unable to run First Time Setup, see also: Remove Duplicated Fields from Object List query
Run First Time Setup
After running First Time Setup, the new object Query will contain the corrected API Name of the field and the Error will be resolved. If the Error persists this means you have a duplicate field on a related Object. Check the other Objects in your system for the Field with a Duplicate API Name.
Note: If the Duplicate Field Name cannot be changed due to the Field being part of an Apex Trigger (see screenshot below), you will still REMOVE the Field Name from the Object List Query record, but do NOT run First Time Setup. Running First Time Setup will put the Field API Name back in the Object List query and the error will persist if the duplicate field is being referenced within the Object List fields.