Duncan is one of three youngsters being linked with the Black Cats, along with defender Ibrahima Mbaye and attacking midfielder Daniel Bessa.
Ghana international Duncan has been one of Sunderland’s prime targets this summer, but Inter are holding out for an offer around the £7m mark for the 20-year-old.
Sunderland director of football Roberto De Fanti held talks with Inter sporting director Piero Ausilio on Friday over Duncan, and the Wearsiders are said to have seen an improved bid rejected.