Together with Wolfgang Kieling, who dubbed Bert, they originated the duo on German language TV. His voice was so closely associated with Ernie that, in 1994, when original puppets of Ernie and Bert were stolen from a touring Muppets exhibit, Duwner made a radio appeal on January 25, asking the thieves to return the puppets. The strategy brought results by March, when radio station Antenna Tueringen received an anonymous phone call to return the puppets, and accepted them through a middleman the next day. He even dubbed Ernie in other movies which used his voice only in passing, such as Where's Poppa? or Short Circuit 2 (with Ernie replacing background audio of Mister Rogers' Neighborhood).