Central defender Duffy is a former Welsh international having been capped 13 times for his country.
The 31-year-old started his career at Swansea City before moving on to Portsmouth, where he failed to break into the first team on a regular basis and got his experience out on loan to Burnley, Swansea and Coventry.
After spending half a season at Millwall, the defender settled at Exeter City, spending three seasons with the Grecians in League One.
He left St James Park to join Port Vale and helped the Valiants to win promotion from League Two during his first season in Staffordshire before establishing them as a League One outfit.
He dropped down two divisions to join Eastleigh but left without making an appearance to join Notts in June 2016.
Home kit: Nottingham Bierkeller
Away kit: Karen and the late Gerald Knight
Third kit: Available
Boots & socks: Available