It seems to be failing sometimes when you insert identical rows in parallel. I think this is because the insertId is being generated from a stringified md5 hash of the row. Would you be open to a pull request to pass in your own insert id in an optional metadata object? It could fallback to how it operates now.

https://github.com/GoogleCloudPlatform/gcloud-node/blob/fbf1ecc36c45c48aeee5469e9729a529d94ee610/lib/bigquery/table.js#L904