Hi there, the duplicate photos on your device might be the synced files if you enabled iTunes sync previously. To delete synced photos from your iPhone, you have to sync again with iTunes and deselect the synced albums in iTunes. Alternatively, you can enable iCloud Photos on your iPhone. iCloud Photos is not compatible with iTunes Photos Syncing and will erase all photos from the iPhone that have been synced with iTunes. Once the iTunes photo has been erased, you can disable iCloud Photos again on the iPhone.
Take care~
