Scan Library does a full and comprehensive scan of all images in your library and performs ImageIQ on potential duplicates.
Each image is compared to every other image. There's a lot going on, so if you have a large number of images in your library you'll have to be patient.

It is irrelevant what sort order the images are presented to ADD. Once complete, you'll have a full set of date, filename, and hash duplicates ready for you to process for deletion.

Lastly, there are a few advanced processing options that might interest you. They are:
1. Ability to tag Unique images.
2. Ability to propagate ratings amongst sets of hash duplicates.
3. Ability to propagate comments amongst sets of hash duplicates.