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Mayor Bill de Blasio has one possible solution to this year’s spate of cyclist fatalities: helmet requirements for Citi Bikers.

“I have thought about that,” he said in response to a question at September’s monthly crime stats press conference Wednesday. “That’s something we’re talking about inside the administration.”

There have been 21 cyclists killed in New York City since the start of the year, including one Citi Bike rider.

Yet while a helmet may be a “no-brainer” for most people, the safety benefits have been disputed. British psychology researcher Ian Walker has found that helmets encourage risky and dangerous road behavior by both cyclists and adjacent drivers.

A 2016 study by Toole Design Group, meanwhile, analyzed data from eight different countries and found that while the US had the highest rates of helmet use — 55 percent — it also had the highest rates of cyclist fatalities.

But another 2016 study out of Australia found that helmet use around the world reduces the chance of fatal head injuries by 65 percent.

De Blasio also told reporters he was open to requiring licenses and registration for bicyclists.

Bike New York Communications Director Jon Orcutt, who helped launched Citi Bike as DOT Policy Director during the Bloomberg administration, called both ideas “counterintuitive.”

“If you put additional requirements on people and subject them to additional enforcement, they will bike less,” Orcutt said. “You don’t want to discourage cycling. The more cycling there is, the safer it gets.”

City Council Speaker Corey Johnson echoed that argument, tweeting that the proposals would be an “obstacle” to safer cycling.

“These ideas won’t help us increase bike riding in NYC,” Johnson said.

There’s also the question of who would provide the helmets: In Vancouver, where helmets are mandatory for all cyclists, the local bike-share company provides riders with the safety accessories — as well as disposable hairnets.

“There are different viewpoints on what would work and how much impact it would have,” de Blasio told reporters. “I think the big questions are: Is this something we could actually enforce effectively? Would it encourage or discourage people from riding bikes?”

“I care first and foremost about safety, so we’re having that discussion right now,” he added. “It’s a really valid issue.”