Sunderland linked with summer swoop for Republic of Ireland international
The 24-year-old is a Republic of Ireland international and is valued at the same amount as January signing Lamine Kone, who joined from Lorient.
Duffy, who has been a Rovers player for two years, is reported to be under contract at Ewood Park until the summer of June 2017.
The former Everton youth has been a regular for Championship side Rovers, who currently have Danny Graham and Jordi Gomez on loan from the Black Cats.
Allardyce strengthened his defence with the signing of Kone, but, with Wes Brown expected to leave Sunderland in the summer, Allardyce will be looking to bolster that area of his squad.
Meanwhile, Sunderland have also been linked with a close-season move for Bordeaux midfielder Clement Chantome.
The 28-year-old is out of contract in the summer and is free to talk to other clubs.
But the Black Cats, one of nine clubs who had scouts at Bordeaux’s goalless draw with Nice on Friday night, face stiff competition to land versatile Chantome.
Leicester City, Arsenal, Tottenham, Manchester United are among a host of clubs linked with a move for the Frenchman.
Chantome, a former teammate of Sunderland’s £9million forward Wahbi Khazri, is a defensive midfielder by trade but is also capable of playing out wide or at centre-back.
Experienced Chantome, who made more than 160 appearances for PSG, has one international cap for France.
He was first linked with a move to the Stadium of Light back in 2013.