Syncronizing two picture photo folders windows to mac

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Here is the problem

1) My Father likes to buy computers, if he has trouble with one he buys anothe.
2) computer 1: HP laptop windows 7 and was putting all his photos there (he takes a lot of photos)
3) computer 2: iMac mac osx and started putting them there
4) then started putting some one one and some on the other, sometimes putting on both, sometimes not.
5) the good news is he rarely changed the filename it is usually of the form img_xxxx and they are the same size.
6) the bad news is the files seem to have different timestamp and the folders all have different names.

What I want is to keep the iMac as the master and backup to the HP, but I need to first merge all the photos into one folder and get rid of the duplicates. there are 600GB of photos on the iMac and 500GB on the HP.

Could freefilesync do this? if so can someone point me in the right direction.