Hi Mike,
Thank you for the details!
I tried to debug this for over an hour, but I could not find any issues within the search side. The query variables are properly set, even the results in the query are correctly passed and set, there are no duplicates in the final results.
Somewhere along the ECS the duplication happens – try checking the settings there, something along custom query arguments or similar, those should affect the final outcome.
I tried to inspect their source code, but I am not familiar with those internal workings, so I could not figure out where the issue might be.