[1] Born in Stewartstown, County Tyrone, Duffy studied at St Patrick's Academy, Dungannon and then Queen's University Belfast before becoming a solicitor.
[2] He became politically active in the Nationalist Party,[3] then in the Unity movement, acting as agent for Frank McManus, the successful candidate in Fermanagh and South Tyrone at the 1970 general election.
[2] After the election, Duffy was a key founder member of the Social Democratic and Labour Party (SDLP), and served as its first treasurer.
[5] In the late 1970s, Duffy became known for his outspoken support of Northern Irish independence,[6] although he did concede that there would need to be some form of federal structure covering the entire island.
[3][10] Outside politics, Duffy built up a large legal practice, with offices in small towns across Northern Ireland.