I've run through the issue as per the article https://our.umbraco.org/documentation/Add-ons/UmbracoCourier/DataIntegrity however no duplicates are being returned and nor any other SQL queries that I've crafted so far for similar data rows.

What other situations could this error response be generated for in Courier?

Our Umbraco database has some issues, one of which is ghost node entries in the cache which due to the daft way the backend UI reads from the cache and not the actual database is causing URL path issues (seriously: backend edits should always work on the master data, not a copy). I'm trying to find the cause of these issues and one thing I'd like to do is to make a copy of the live environment using Courier before breaking making data changes that could break it further... however I can't do this due to the above error. I'm not surprised, as I'm using Courier as a tool to try and help find or even fix these problems but some form of useful indicator as to the actual problem rather than a bland error message would be most helpful. Not ideal but it's what I have to work with right now...