hey alex,
started playing with the new 6.0.4 release on the latest easysocial 1.2.7 and have good news and bad news for you
the good news: the detection of cover photos seems to work nice now, no more duplicates so far.
the bad news: the cover photo is not created correctly (easysocial shows an empty image, checking the path to the image media/com_easysocial... it's not there at all)
I then digged deeper to see if the pic was catched from facebook and looking in the tmp folder it seems to be there correctly (sccover_facebook....tmp.jpg)
so it seems that in the step from fetching the image to the creation of the different variations for easysocial and copying over to the easysocial media folder something's wrong at the moment.