Duncan is a litigation lawyer described in Legal 500, the leading peer and client review directory of recommended lawyers, as “indefatigable” and is named as “the client’s champion.” Duncan was Chairman of the McNair Inquiry and Report (2013) on the welfare of farmed animals, commissioned by the RSPCA. In 2017 Duncan was part of a key group representing 220 charities and MPs in presenting a letter to the Prime Minister, demanding abolition of the UK ivory trade. A proposed ban was announced immediately. In 2018 Duncan was named Winner of the Animal Hero of the Year award at the Animal Star Awards.

In 2024 Duncan was named Legal Hero of the Year by the Law Society out of 500 nominations drawn from 200,000 solicitors. This recognised his devising of the Animals (Low-Welfare Activities Abroad) Act and his leading role in ensuring its passage through Parliament and its global implications for animal welfare.