(covers information from several alternate timelines)
Lieutenant junior grade Duffy was a Starfleet engineer stationed on board the USS Enterprise-D in 2366. While the starship was on the way to Nahmi IV, he was ordered to realign the magnetic capacitors together with his colleague Costa. During the mission he was contaminated with invidium and subsequently spread this element throughout several ship's systems. While drinking a beer in Ten Forward, he contaminated his glass with the invidium and thereby changed its molecular structure, causing it to leak. (TNG: "Hollow Pursuits")
In an alternate 2369 experienced by Jean-Luc Picard after his death in that year, Chief Engineer Geordi La Forge told another engineer that Duffy was to adjust the main deflector and look into an inducer problem. (TNG: "Tapestry")
In Pocket Books non-canon Star Trek: Starfleet Corps of Engineers series, Duffy appears (in the post-Dominion War era) as the second officer of the series' ship, USS da Vinci, rank of lieutenant commander. In the books, his first name is Kieran, which coincidentally is also MacDuff's first name. When the da Vinci became trapped in the gas giant, Galvan IV, Duffy gave his life to complete his mission and ensure the safety of his crewmates.