Following on from my blog post yesterday I spent most of last night firing a handful of emails back to Jason McCay at MongoHQ working through this oddment issue.

In the end, and with some help from Brendan McAdam’s who maintains the MongoDB JDBC driver, we think it may have been down to a previous version of the driver.  I cloned the repository from Github and compiled it myself and the latest version worked first time with no fault.  One you can do is for a safe write with WriteConcern.SAFE which will attempt to make sure everything is nice and clean in terms of the write and attempt to avoid any concurency issues.