import from Android should not include thumbnails
When using MTP to import photos from an Android phone, shotwell also imports the thumbnails that are generated by the default "gallery" app. Those thumbnails are stored in a folder named .thumbnails
which resides in the DCIM
folder.
Alternatively shotwell should not import images from hidden "dot" folders at all (or offer an option to do so, e.g. a checkbox "include hidden folders").