The visitors looked the better side for much of Tuesday’s League One clash at the Memorial Stadium and hit the woodwork twice through Richard Logan in the first half and man of the match Alex Russell in the second half.
But the key moment came in the 33rd minute when Jo Kuffour’s low shot was saved by Andy Marriott, whose touch left Duffy with an open goal from inside the six-yard box.
The goal was rough on Exeter, who had passed the ball more accurately in difficult conditions. A strong wind and swirling rain made life difficult for both sets of players.
Rovers goalkeeper Mikkel Andersen made a fine 70th-minute save from Ryan Harley’s 25-yard volley as Exeter poured forward in search of the equaliser.
But Rovers defended strongly against the wind in the second half and restricted their opponents to few clear-cut chances, while Exeter goalkeeper Marriott had his best moment in the 24th minute when palming away a stinging shot from Mark Wright.