Thank you Yastiandrie !
This was the first thing I asked when I contacted Telstra last week as I had just had an update run for my 2016 Office apps. And I had a suspicion that this may have been the cause. But I was told the consultant had checked and it was a telstra error....
When this first happened, my emails on my phone kept disappearing and it turns out they were all deleted off the server. The consultant played around with the settings, turned the delete off server option off, which I would say, stopped the emails being deleted off my phone. But the duplication of the emails continued and this appears to have increased.
I will follow the microsoft recommendations and report back.
btw it is not appropriate to ask customers for their passwords and it is even less appropriate to tell customers that if they don't give some unknown faceless person their password then they are not going to get their problem fixed. This issue was never going to be resolved that way was it ?