WASHINGTON — They brought the gun show to the airport.

Passengers at Ronald Reagan National Airport were given an unexpected surprise Monday when fitness buff Health and Human Services Secretary Robert F. Kennedy Jr. got into a pull-up contest with Transportation Secretary Sean Duffy while flexing a billion-dollar investment.

Before testing their strength, the two cabinet secretaries were touting a $1 billion investment to make airports across the country more family-friendly, conducive to good health and better for breastfeeding.

“I don’t want you to have a full-body sweat going on till you stink,” Duffy told The Post after taking off his tie and jacket to pump out a set of pull-ups when asked about concerns that gym nuts will assail fellow travelers with workout funk.

“But if you

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