Photo deDupe needs access to original-quality photos in order to function and effectively check for duplicate media. This means that original-quality files must be synced to the device you're using to Run Photo DeDupe OR stored on a physically connected external drive.
If you launch Photo DeDupe and see a yellow border around your image sets, along with the error "All originals for a duplicate set must be present to automatically deduplicate media," this means that Mylio Photos does not have access to the original-quality files.
To fix this:
Physically connect an external hard drive (such as a Vault) where the original-quality media is stored.
Temporarily download the original-quality files to the device you're using.
On a computer, select the files you wish to download, right-click, and choose Download > Originals.
On a tablet, click the Actions menu in the bottom right corner and choose Select > Some. Tap on the images you wish to download, then return to the Actions menu and choose Download > Originals.