I work in a bike shop in Holland and I see at least two bikes a week that have broken spokes or collapsed rear wheels because the customer carried someone on the rear rack. Whenever I tell that customer that bikes were never designed to carry an extra passenger, I always get puzzled looks and answers like: 'but I thought my bicycle was made to do just that' , or 'well I've always carried people on the back and never had a problem before!'
Dutch people are very spoiled when it comes to this issue and that is because many of the bicycles that were made 20, 30 or 40 years ago ( and many of those are still running) were made significantly sturdier than the bikes produced today. The only way to do it now would be to install a heavy duty rear rack or buy a bike like the Batavus Personal Bike.