The calculated distance (air line) between Duisburg and Alphen aan den Rijn is approximately 102 miles respectively 165 kilometers.
By car or train, the actual journey to Alphen aan den Rijn is certainly longer, as only the direct route (as the crow flies) between Duisburg and Alphen aan den Rijn has been calculated here.
Air line (approximately)
165 kilometers
89 nautical miles