We’ve been looking into this and we have not yet been able to reproduce the problem with either of the printers we have here in our London office, either on Windows or Mac. I think both of you are on Windows, yes?
Is it possible for you to try a different printer driver for your printers? I know that often the generic Microsoft drivers don’t provide support for special features like duplexing, but if it’s possible for you to give it a try I’d be interested to know if this makes any difference.