A 41-year-old senator, Duque faces his leftist rival Gustavo Petro on Sunday after comfortably winning the first round. Opinion polls give him a margin of between six and 15 points over Petro.
He campaigned on a ticket to rewrite the peace deal signed with the FARC by outgoing president Juan Manuel Santos.
A lawyer with a degree in economics, Duque represents many Colombian voters who were outraged by concessions given to the former rebels, including reduced sentences for those who confessed their crimes.
If elected, he has promised to make "structural changes" to the 2016 agreement, which led to the group's disarmament and conversion into a political party.
"What we Colombians want is that those who have committed crimes against humanity be punished by proportional penalties... so that there is no impunity," Duque told AFP during the campaign.
Latin America's longest-running conflict left more than 260,000 people dead, nearly 83,000 missing and some 7.4 million forced from their homes.
Uribe puppet?
Duque has railed against the Colombian left, voicing fears that they would drag the country into the same economic quagmire in which neighboring Venezuela is mired.
The left in turn accuse him of being a puppet of Alvaro Uribe, the former two-term president who took a hard line against the left when he was last in power eight years ago.
"Nobody knows if he has his own criteria or if he will obey orders," Fabian Acuna, a political analyst at Cali's Javeriana University, said of Duque.
Although a newcomer to politics -- he has been a senator since 2014 -- politics is in his blood.
Born in Bogota on August 1, 1976, his father was a liberal politician.
But it was Santos, the outgoing president, who took him under his wing in the 1990s as a financial advisor. Later, he worked for 13 years for the Washington-based Inter-American Development Bank.
Today, Duque finds himself in opposition to Santos over the peace deal.
"He is very dynamic when it comes to public relations, very clever," said a former co-worker at the IDB.
While working in the United States, Duque met Uribe, who persuaded him to run for Colombia's Senate.
"Ivan is very intelligent and I'm sure he has a bright future ahead of him," wrote Uribe in his 2012 book "No Lost Causes."
But for Roy Barreras, a senator from Santos' party, "a president must have experience, autonomy, political capacity -- all missing with Ivan, who is, as everyone admits, a good little guy."
A father of three, Duque used to play bass in a rock band, but his relaxed image contrasts sharply with his conservative ideals -- he is a staunch opponent of gay marriage, euthanasia and the decriminalization of drugs.
He has strong support from the far-right as well as an increasingly influential evangelical Christian bloc.
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