May’s double took his tally to 20 for the season and 10 in his last seven games.
The 28-year-old left Rovers for the Robins in January 2020 and is on course to become their all-time record goalscorer in the EFL.
Duff was asked after his side’s 4-0 win whether he saw a 20 goals a season striker in May when he signed him.
He replied: "Course I did!
“We felt that when he played at Doncaster, they played 4-3-3 and they stuck him out on the wing.
“Everyone knows what he is, he's infectious and we thought he could become a goalscorer.
“Off the back of last year, you are probably thinking no because we've won the league and he's scored nine goals.
“But he's become so much more than that and Kion (Etete) has to take a lot of credit for the glut of goals he has, which is from playing with Kion.”