Two years ago, on a chilly, gray, rainy Saturday afternoon in London, Anaïs Bordier received a screenshot, sent by a friend, of a young woman in America who looked exactly like her.

“I was shocked,” Anaïs says today. “I didn’t understand ... who could that be?” Electrified, her next thought was to find out more about this girl. And just as quickly, she dropped the idea.

Anaïs was adopted as an infant from South Korea and raised as an only child in Paris and Brussels. She knew nothing about her biological mother — only that her adoption papers listed her as a single live birth. That’s why, Anaïs says, “I didn’t think she could be my twin. But the resemblance was so strong that I thought she might be related to me to a lesser degree — like my cousin or something.”

What are the chances you find someone adopted from the same country, same town, who was born on the same day?

 - Anaïs Bordier
Meanwhile, her friends poked around the Web and found the same young woman — a 25-year-old actress named Samantha Futerman — in an online trailer for a movie called “21 & Over.” This time, Anaïs investigated.

“I found her date of birth,” she says. “What are the chances you find someone adopted from the same country, same town, who was born on the same day?”

She thought of “The Parent Trap,” starring Lindsay Lohan as a tweenage girl who discovers her long-lost identical twin.

“So this time, I had no fear,” Anaïs says. “I thought we could be twins, and twins have a very strong bond. So I made the sudden decision to message her.”

It was Feb. 21, 2013. She sent Samantha a friend request on Facebook that read, in part:

“I stalked you a bit and found out you were born on the 19th of November 1987 ... and discovered you were adopted too. So ... I don’t want to be too Lindsay Lohan, well ... but ... how to put it ... I was wondering where you were born?”

And then, Anaïs waited.

In their new book, “Separated @ Birth: A True Love Story of Twin Sisters Reunited” (G.P. Putnam’s Sons), Anaïs and Sam tell their stories, in alternating chapters, of life apart and, now, together. It has been only 18 months since they’ve been in contact, and when Sam got Anaïs’ first message, she was unmoored.

“I felt shock and awe,” Sam says. “When I saw where she was born and adopted out of ... I think I was just taking it all in.”

Sam couldn’t bring herself to respond immediately, but in her bones, she already knew.

“That entire day, I’d just keep randomly turning to my friends, saying, ‘I have a twin! I have a twin!’ ”

It took Sam two or three days to reply. Anaïs was going crazy.

“It was extremely stressful,” she says.

Seeing Anaïs on Skype was unreal … She had my laugh, my freckles, and that profile … I stopped for a second and freaked out inside.

 - Sam Futerman
Sam was still absorbing the possibility that this was her twin and was so emotionally overwhelmed that she shut down. “I held off for a little bit,” she says. Her parents, who raised Sam in Verona, NJ, were skeptical. Football player Manti Te’o was all over the news, caught in a scandal over whether he knew his online girlfriend had been a hoax all along. Her father worried Sam might be a similar target of so-called “catfishing.”

Eventually, Sam’s curiosity won out, and on Feb. 26, they had their first Skype session: Anaïs in London, where she was studying fashion design at Central St. Martins, and Sam in LA, where she was working as a waitress/actress.

“Seeing Anaïs on Skype was unreal,” Sam writes. “I had never seen anyone who looked even remotely like me, let alone my exact mirror reflection. She had my laugh, my freckles, and that profile. When she turned to the side during that first Skype session, I was blown away. I stopped for a second and freaked out inside.”

For Anaïs, that Skype session was all the proof she needed — especially when they compared baby pictures: They had the same expressions. They learned that throughout their girlhoods, they had had the same series of haircuts in the same order. They both hate cooked vegetables, carrots especially. They both have the same manner of speaking, trailing off mid-sentence. They both brush their teeth multiple times a day, have a fear of being grazed by a shower curtain, freely admit to Napoleon complexes, and require 10 hours of sleep a night, plus daily naps.

They were supposed to talk for 90 minutes and wound up going for three hours.

“I wanted to see her in person right away,” Sam says.

“I wanted to get on a plane immediately,” Anaïs says. They took a DNA test, only to prove to ­others what they already knew.

It would be two months before they were in the same room. Both sets of parents were happy for the girls, but livid that the adoption agency in Seoul had lied, had separated the girls, had robbed them of 25 years they could have had together.

“My parents were angry, for us and for themselves,” Anaïs says. “They would have been so happy — both our parents — to raise twins at the same time.”

Not much is known about adoption practices in South Korea in the ’80s, but single mothers there face a stigma and will disappear for months to “birth houses,” where they hide until they give up their babies for adoption.

Sam immediately got in touch with the agency in New York that brokered her adoption. While she and Anaïs waited for the results of their DNA test, she learned that the equivalent of her birth mother’s Social Security number also appeared on Anaïs’ papers, and that they had both been born at the same clinic in the city of Busan. The clinic had long since closed, and the doctor who delivered them had passed away.

A caseworker contacted the woman listed as the biological mother. She denied ever having given birth to them.

Anaïs doesn’t think about that too much. “I put it aside,” she says. “I wasn’t disappointed, but I wasn’t relieved, either. I have no idea who she is or what she does. I put it aside, and we’ll see what happens in the future. Maybe she’ll contact us. Maybe she never will.”

Sam, however, thinks about her birth mother often. “It’s a bit overwhelming to hear that someone you think you might love wouldn’t be reciprocating,” she says. “I also feel an immense sadness for her. That she would feel so much guilt or sadness or pain that she would have to deny us ... My sister and I do love our birth mother. She gave us life.”

On May 13, 2013, Sam flew to London to meet her twin sister. She had spent the last two months getting funding together for a documentary about their separation and reunion, and she had timed her visit to Anaïs’ graduating fashion show. She drove to Anaïs’ flat in Shoreditch, the documentary crew, family and friends all acting as buffers.

When she opened the door and entered the room, the two girls stood apart, silent, staring at each other. Someone yelled at them to hug, but neither girl could move.

“It was very strange,” Anaïs says. “Physically, very strange. I would describe it as opposing magnets attracting each other. It’s like seeing a mirror that doesn’t react the way it should.”

She crossed the room and poked Sam in the forehead. Anaïs says she felt like she was in a sci-fi movie or an alternate reality, and literally thought that if they hugged, “an explosion might happen. I felt like we were in two parallel rooms and we shouldn’t be standing in the same place. I needed to make physical contact with her, to check that I wasn’t dreaming. And when I poked her, I knew that she was real.”

It was very strange … It’s like seeing a mirror that doesn’t react the way it should.

 - Anais
Sam burst into laughter; she wasn’t that surprised. In the weeks since she and Anaïs made contact, she had been working with Dr. Nancy Segal, director of the Twin Studies Center at California State University-Fullerton. “I’ve seen videos of other long-lost twins reunited,” she says, and poking is commonplace. “It’s a safe distance to be away from someone but confirm that they’re real.”

After the reunion, the entire group went out to lunch, then left Anaïs and Sam alone. They went back to the Shoreditch flat to take a nap, sleeping side by side. “Maybe this was our way of resuming our story where it started — twins in the womb,” Sam writes. “We were resuming our life together, waking up with no fear of ever being separated again.”

That night, they got the results of their DNA test, and to no one’s surprise, they were, in fact, identical twins. It was Segal who delivered the news, and she was uniquely qualified to help the girls navigate this new reality: She had been a lead researcher in a 20-year study called the “University of Minnesota Study of Twins Reared Apart.” One hundred thirty sets of such twins had participated, and one of the most groundbreaking findings was the dominance of nature over nurture when it comes to intelligence, athleticism, vices, partner preferences — even hair length.

Anaïs and Sam have each undergone testing by Segal, and even they were stunned by their similarities. “It tells you a lot about human nature,” Sam says. “I thought everything is nurture, but a lot of it is nature. All of our cognitive abilities are exactly the same — when you look at the data, they are parallel.”

In the months after their reunion, Sam and Anaïs have learned of two other sets of twins born in South Korea, separated at birth and reunited later in life. They also met a third pair of South Korean-born siblings who not only found each other but discovered that they are actually triplets, and that their birth parents kept their lost sibling.

And one month after Sam found Anaïs, she got an email from her friend Dan, also a South Korean adoptee. He had just begun his own birth search a few weeks before. “I’m a twin, too,” he wrote. “I’m not even f--king kidding.”

Anaïs and Sam both hope their story may help other long-lost siblings find each other. Along with “Glee” star Jenna Ushkowitz — herself adopted from South Korea at age 3 and raised in East Meadow, LI — Sam has co-founded Kindred, an organization that aims to give financial and emotional resources to adoptees and orphans worldwide. Their documentary will make the festival rounds in the next few months, and this week they’ll see each other for just the eighth time ever, in New York.

They talk all day, every day about the important and the mundane. A typical text from Anaïs, says Sam: “I want to drink my tea, but I can’t because it’s too hot.”

Today, Anaïs lives in Paris and Sam in LA, but they hope soon, somehow, to live in the same city.

“All I want from my sister is to spend time with her, to call her after work and say, ‘Hey, want to get dinner?’ and to see her weird habits,” says Anaïs. “We are starting our life and going in the same direction. Ours is a love story, but it’s a family love story.”

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