When importing from catalog, it ́s not possible to select images. If they already exist in the catalog.
1. Importing pics from sdcard to WIN10-PC
2. Importing same pics from sdcard to iMac Pro
3. Develop pics on iMac Pro
4. Export developed files as catalog on USB_Stick (without copying the dngs)
5. Import from catalog from USB_Stick to WIN10-PC
6. Select all pics and overwrite
With LR 2015.6. this is not possible anymore, because the files are not selectable. Up to 2015.5 this works fine.