Let me ask something else... suppose as my main pic/video tank i have my iMac and i delete the photos from my iPhone, is it possible that sometime there can be some kind of sync and the iPhone photos is duplicated on my main pic/video tank which is my iMac. That way as you understand i will have a problem!!!
It should only import the new photos, if I'm understanding your question correctly. There is of course the possibility of some such error happening that duplicates photos, but I've not heard of it before. And with duplicate photos (or any file, for that matter) situations, I would be wary of deleting any files before making a backup - sometimes files appear to be duplicated but are not, and then you've deleted the originals.