when you download photos if you have duplicates and check skip its skips photos, even photos wich are not duplicates
If you transfer photod and then come back the next day to transfer any new photos, if all photos are checked and you mark transfer there is a box wich says duplicate filename and asks replace, rename or skip and if you check skip and also check apply to all
it skips all photos (not just the duplicates) and does not transfer any phtos, so
in order to update any new photos you have to determine wich phots have been previously transferred and uncheck the selection box.