How to duplicate-check between iPhoto Library and Photos Library *and* move non-duplicates from iPhoto -> Photos?
Perhaps a little bit late for esaruoho's friend but might be useful for someone else.

FatCatSoftware, who created the iPhoto Library Manager that esaruoho linked above, have created a similar application for managing Photos.app libraries called PowerPhotos.

For this situation I would go with option 1. 


Convert the iPhoto Library to a Photos.app library.
Combine the two Photos.app libraries into one by using PowerPhotos.
Run a duplicate image check using PowerPhotos in order to make sure there are no duplicates.
Upload images to iCloud.