Awesome photoFinder is reasonable fast, but...
Option 1:
Say I have two folders A and B
A has 100 pictures, B has 10 pictures
I want to know if pictures from B are in folder A, so (10x100=) 1000 comparisons.
PhotoFinder also looks for duplicates WITHIN folder A and B,
so (110x110=) 12100 comparisons.
Second option:
Pictures LOOK similar/identical but the meta data/dates etc. is different.
Flag if that is only difference.