I and my whole team encounter this error while inserting some data into a new Scratch Org.
DUPLICATE_VALUE, duplicate value found: <unknown> duplicates value on record with id: <unknown>: 
What we are doing:
* create a new scratch org
* push source
* insert one account or one custom object (UI/Apex doesn't matter)
* insertion usually fails even though no records are in the scratch org
The problem is it's indeterministic. Tried the same steps more times on the same source, the same devhub, and sometimes it succeeds, sometimes it fails.
Moreover, another team working on a different project is not facing this issue at all.
DevHub: release 19'
Source worked as expected. On 5th September it stopped working, i.e. newly created scratch org started to throw this exception even though no changes in the source were made. Even tried to create scratch org from old source that was validated through our pipeline at that time and now it's facing the same issue.