Duffy rescues Ireland with late leveller in Denmark draw
Brighton defender Duffy headed home with just five minutes left at the Telia Parken.
Southampton midfielder Pierre-Emile Hojbjerg had come off the bench to put the hosts ahead with 14 minutes left.
Duffy's last-gasp leveller left leaders Ireland with seven points from their first three Group D matches following wins over Georgia and Gibraltar.
It was Denmark's second successive draw after they were held by Switzerland in their opening qualifier.
Ireland coach Mick McCarthy had said he would be "thrilled with a draw" and he got his wish thanks to a gritty display that saw his side survive periods of intense Denmark pressure.
Denmark threatened first when Christen Eriksen unleashed a fierce free-kick that bounced awkwardly in front of Ireland keeper Darren Randolph.
Ireland almost snatched the lead when Conor Hourihane's free-kick was headed goalwards by Duffy, only for Denmark keeper Kasper Schmeichel to save on the goalline.
Tottenham play-maker Eriksen sparked talk of a transfer recently when the Dane said he "might want to try something new".
Playing for the first time since Tottenham were beaten in the Champions League final by Liverpool last weekend, Eriksen was pulling the strings for Denmark and Thomas Delaney headed just wide from his corner.
Eriksen scored a hat-trick against the Irish in the 2018 World Cup play-offs and early in the second half he almost conjured the opener when he sent Yussuf Poulsen through on goal with a superb pass.
Poulsen's nimble touch took him past Duffy, but Randolph came to Ireland's rescue with a fine save.
Ireland burst into life when Robbie Brady picked out James McClean for a chance that Schmeichel saved well at his near post.
When Martin Braithwaite fired wastefully over, Denmark boss Age Hareide shouted in frustration on the touchline.
Delaney fared no better with a misguided header from Henrik Dalsgaard's cross.
After all that angst, Denmark's pressure was rewarded in the 76th minute.
Just four minutes after his introduction, Hojbjerg beat Richard Keogh to the ball and headed Jens Stryger Larsen's cross past Randolph.
But there was a sting in the tale for the Danes as Duffy equalised in the 85th minute.
Alan Judge whipped in a fine free-kick and Duffy powered a header past Schmeichel for his third international goal.
McClean could have won it for Ireland in the final seconds, but his effort was repelled by Schmeichel.