We recently upgraded from mongo-java-driver 2.9.0 to 2.11.2 and started getting DuplicateKey exceptions in a scenario where we do a remove for a key followed by an insert.

All of our operations use WriteConcern.SAFE, so they should be serialized and the DuplicateKey exception should be impossible.

I dug into the code and found the following change in https://github.com/mongodb/mongo-java-driver/commit/b4d0a6ee933b268f9ca0b890adc4aed1bf30e097 :

• if ( _w instanceof Integer && ( (Integer) _w > 0) ||
• ( _w instanceof String && _w != null ) ){
  + if (_w instanceof Integer && ((Integer) _w > 1) || (_w instanceof String)) { _command.put( "w" , _w ); + }

It looks to me like the driver is /not/ sending 'w' with the GLE command when 'w' is 1 (which corresponds to WriteConcern.SAFE or WriteConcern.ACKNOWLEDGED). But I believe it should be. If it doesn't send 'w', mongod does not wait for the previous operation to finish. So we are effectively getting WriteConcern.NORMAL instead.