As many others have already done, I am providing yet another cautionary tale of the elusive, misleading, and downright frustrating, magical error:
duplicate value found: <unknown> duplicates value on record with id: <unknown>
- Attempting to deploy an Apex Class from my local IDE
- Deployment fails, no error messages back whatsoever
- Created the empty Apex Class directly in the Salesforce Developer Console
- Pasted in content from local IDE
- Attempted to save, gets back the magical error message in the Problems list
- Searched online, everyone is saying one of the following:
- It's version history on fields, turn it off, apparently this one is solved as of Winter '19 release
- An update DML statement is violating some kind of uniqueness constraints.
- Commented out class body, no effect
- So the issue isn't update DML statements either.
- Commented out superclass extend, no effect
I have posted the resolution I found below as an answer.