| Jack Duncan | |
| Birth Name: | John L. Duncan[1] |
| Birth Date: | c. 1900 |
| Birth Place: | Sydney |
| Death Date: | c. 1969 |
| Ru Position: | scrum-half |
| Amatteam1: | Randwick |
| Repyears1: | 1926 |
| Repteam1: | Wallabies |
| Repcaps1: | 1 |
| Reppoints1: | 0 |
John L. Duncan (c. 1900 – c. 1969) was a rugby union player who represented Australia.
Duncan, a scrum-half, was born in Sydney and claimed 1 international rugby cap for Australia.