When the powers-that-be saw it they decided they wanted Duel as a regular, so it would have been Duel and Davis. Then someone spotted Ben Murphy and decided his resemblance to Paul Newman would attract a lot of attention from feminine viewers, so Roger was left out altogether and it became Duel and Murphy. Considering the fact that Peter absolutely hated working in a series, he'd probably be very happy to know Roger has his original job back.