I think what is happening is that the tag has its last value stored in history then the device loses connection (On Change triggered), it then logs the same value but with bad quality then the device reconnects, on change is triggered again and it logs the same value with good quality again. I will verify if this is the case on Monday.
I cam across this http://forum.inductiveautomation.com/t/table-tag-history-and-bad-quality/4319
But even if I could filter out the bad quality, I will still have consecutive duplicates each time the device loses connection and reconnects (just one less).