When I try to redeploy an object that exists in source and destination orgs, the deploy fails with the cryptic and useless error message:
duplicate value found <unknown> duplicates value on record with id: <unknown>
This fails whether I deploy with Migration Tool or IDE.
Note, individual fields are deployable, but not the object.
Best Answer
Editor's Note: This issue has been resolved in Winter 19
Figured out the problem. Some Salesforce issue with History tracking was the root cause. Turned off history tracking in dest org and once the history object was eliminated, the object could once again be deployed.
Not sure why there was an issue, though the object label was changed in the source org. I think SFDC was trying to recreate the History Object during the object deploy. I would have to say there's an underlying Salesforce Bug that should be resolved here, but a 2 minute workaround is better than the full day lost having to go through their support process.