The right back has signed to provide cover for Isaac Osbourne for the rest of the season.
Duffy had returned to Portsmouth last season after a spell at the Ricoh Arena and has not appeared for their first team this season.
The Swansea City supporter, first joined City in 2005 under Micky Adams and has had two more spells at the club, going on to make sixty appearances (plus two sub) and it is believed that his fourth loan to the Sky Blues is a record breaker.
He goes into the squad to face Southampton tomorrow.