Found the following article online, use it to answer the question: What is the last name of the scholar that believes role-doublings were used in the performance of the Montverdi piece?  In his personaggi listed in the 1609 score, Monteverdi unaccountably omits La messaggera (the Messenger), and indicates that the final chorus of shepherds who perform the moresca (Moorish dance) at the opera's end are a separate group (che fecero la moresca nel fine). Little information is available about who sang the various roles in the first performance. A letter published at Mantua in 1612 records that the distinguished tenor and composer Francesco Rasi took part, and it is generally assumed that he sang the title role. Rasi could sing in both the tenor and bass ranges "with exquisite style ... and extraordinary feeling". The involvement in the premiere of a Florentine castrato, Giovanni Gualberto Magli, is confirmed by correspondence between the Gonzaga princes. Magli sang the prologue, Proserpina and possibly one other role, either La messaggera or Speranza. The musicologist and historian Hans Redlich mistakenly allocates Magli to the role of Orfeo.A clue about who played Euridice is contained in a 1608 letter to Duke Vincenzo. It refers to "that little priest who performed the role of Euridice in the Most Serene Prince's Orfeo". This priest was possibly Padre Girolamo Bacchini, a castrato known to have had connections to the Mantuan court in the early 17th century. The Monteverdi scholar Tim Carter speculates that two prominent Mantuan tenors, Pandolfo Grande and Francesco Campagnola may have sung minor roles in the premiere.There are solo parts for four shepherds and three spirits. Carter calculates that through the doubling of roles that the text allows, a total of ten singers—three sopranos, two altos, three tenors and two basses—is required for a performance, with the soloists (except Orfeo) also forming the chorus. Carter's suggested role-doublings include La musica with Euridice, Ninfa with Proserpina and La messaggera with Speranza.
Carter