Duffy released by police
David Black, 52, was shot dead on a motorway on Thursday as he was driving to work at Maghaberry prison.
Mr Duffy, 44, and a 31-year-old man, were detained in Lurgan on Friday.
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Hide AdBoth men have been released unconditionally but a 29-year old man arrested in the Irish Republic on Friday is still being questioned.
He was arrested by the Irish police in County Leitrim and is being questioned in Carrick-on-Shannon.
Mr Black was shot on the M1 on Thursday morning as he drove to work at Maghaberry jail.
Mr Black’s funeral is due to take place on Tuesday.
The Crimestoppers charity has offered up to £10,000 for information leading to the conviction of those responsible for the 52-year-old’s murder.
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Hide AdMr Black, from Cookstown in County Tyrone, was driving to work when a car pulled up beside his vehicle on the motorway in County Armagh and fired a number of shots. His car veered into a ditch and he died at the scene.