DownloadDatabase logs metrics on when history records are dropped. Based on the metrics we see there are a non-negligible number of events where:

 * The download record on disk had a negative ID. Only positive IDs should ever be getting written to the DB.

 * There are multiple records with the same ID. There should be no cases where a duplicate ID is written to the DB.

The UMA in question is:

  Download.DatabaseRecordDropped

The UMA is recorded in:

  void DownloadDatabase::QueryDownloads(std::vector<DownloadRow>* results)

We should have a closer look and figure out what's going on. If the ID field is unreliable, then the other fields are even less reliable.