The calculated distance (air line) between Duisburg and Freyming-Merlebach is approximately 158 miles respectively 254 kilometers.
By car or train, the actual journey to Freyming-Merlebach is certainly longer, as only the direct route (as the crow flies) between Duisburg and Freyming-Merlebach has been calculated here.
Air line (approximately)
254 kilometers
137 nautical miles