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On Sept. 13, Justin Bieber, 24, and Hailey Baldwin, 21, walked into the New York City Marriage Bureau and allegedly got hitched after a two-month engagement.

According to TMZ, the pop star cried at the courthouse, cooing to his model fiancée, “I can’t wait to marry you, baby.”

The hasty wedding was just one of Bieber’s recent eyebrow-raising antics.

In August, he was spotted looking disoriented while shopping shirtless in Tribeca and crying on the West Side Highway bike path the same day. One week after saying “I do,” the singer was busking outside of Buckingham Palace while visiting London with his new wife.

Recently, a video surfaced online of Bieber acting jittery and shaking while meeting fans on the streets of Grand Rapids, Mich., leaving many to speculate that something is very wrong.

It all seems like a step back for the “Sorry” singer, who in recent years has tried to distance himself from the fast life.

After a series of public blunders — a Rio de Janeiro brothel visit (2013), a drug confiscation aboard his tour bus in Sweden (2013) and a DUI arrest (2014) — Bieber sought solace in Hillsong, the Evangelical megachurch that draws 9,000 devotees to its concert-like services at Hammerstein Ballroom in Midtown.

In December 2017, Bieber wrote on Instagram: “Jesus is changing me from the inside out everyday!” But sources close to the pop star worry that his recent erratic behavior is a sign that he’s become unhinged yet again.

One pal of Baldwin told The Post that he used to party with Bieber back in 2014 and 2015 — and that the disturbing viral video of the singer looking wobbly and wide-eyed was all too reminiscent of the bad old days.

“You could tell he wasn’t all there,” the pal recalled of Bieber’s behavior during the 30-person — many of them beautiful young women — gatherings at the singer’s Los Angeles mansion, where cellphone photos were banned. “He would be slurring his words or shaking. You could tell he was not normal.”

Bieber’s longtime manager, Scooter Braun, recently admitted during an interview on “The Red Pill” podcast that at one point, Bieber’s partying got so bad, Braun thought the singer might die.

“I thought he was going to go to sleep one night and have so much crap in his system that he would not wake up the next morning,” Braun said. “There was a time where I would go to sleep almost every night, when he had the money to fly away from me, and I was worried every night that I was going to lose him.”

Bieber seemed to clean up and get healthy when he doubled down on his devotion to Jesus.

Last summer, he abruptly canceled his Purpose World Tour, stating on Instagram, “I wanna be a man that learns from [my mistakes] and grows from them!!” Bieber hid out at the Montclair, NJ, home of Hillsong pastor Carl Lentz instead. It was not the first time he used the place to escape the temptations of Hollywood.

“He’s come a long way,” said New York City-based tattoo artist JonBoy, a friend of Bieber’s who recently inked the singer and Baldwin. “It’s amazing how much he’s changed and how much God has done.”

But as Baldwin’s pal pointed out, it might be a matter of substituting one obsessive habit for another.

“He found restitution through his religion and his faith and that’s where he became super addicted to churches,” the pal said. “He’s super into church [and] Bible study and that’s Hailey’s thing as well.”

Bieber and Baldwin first met backstage at the “Today” show in 2009, when the model’s father, “Flintstones” actor Stephen Baldwin, introduced the two teenagers.

At the time, Bieber had just released his debut EP; it would be a year before he hit it big with “Baby.”

Baldwin, unimpressed, crossed her arms after curtly shaking Bieber’s hand. But never say never.

She eventually warmed up to the crooner, forging a friendship that left his fans speculating about her relationship with Bieber for years, even as he was on and off with fellow pop star Selena Gomez.

And in and out of trouble.

In 2013, Bieber was sued by a photographer for assault and emotional distress after the pop star kicked and punched the man in LA while on a date with Gomez. (The case was settled out of court four years later.) The same year, a video surfaced of Bieber urinating in a mop bucket in a New York City restaurant kitchen.

In January 2014, Bieber vandalized his Calabasas, Calif., neighbor’s mansion with eggs and was ordered to pay more than $80,000. At the time, neighbors also complained about the singer racing his car down the suburban street and about his raging parties — including one at which rapper Lil Za was arrested on suspicion of drug possession.

That same year, Bieber was charged with assaulting a limousine driver in Toronto. He also received a DUI charge while street racing a Lamborghini in Miami and added another arrest after an ATV wreck led to a physical altercation in Stratford, Ontario.

But the bad-boy antics apparently didn’t turn Baldwin off.

In 2016, the two announced they were officially an item with an Instagram post of them smooching. Within seven months, however, they had called it quits.

Behind the scenes, said Baldwin’s pal, the model wasn’t ready to give up.

“She was super obsessed with him. Justin was trying to get back with Selena and [Baldwin] couldn’t get over that. She wouldn’t talk to other guys, wouldn’t date other guys. Wouldn’t give other guys the time of day,” he said. “She wanted to [put] it super out there how religious she was, I think, just so it would hook [Bieber].”

But Baldwin publicly played it cool. While her ex and Gomez briefly rekindled their romance in the fall of 2017, Baldwin began dating Shawn Mendes — who, as a pop star from Canada, has earned many comparisons to a younger Bieber.

If it was a plan to reel Bieber back in, it seemed to work.

In May, Baldwin, who has walked in fashion shows for Dolce & Gabbana and Tommy Hilfiger, told the London Times that she and Bieber were better off as pals.

“We didn’t speak for quite some time [after our 2016 breakup] and there was a lot of weirdness that went on,” she told the newspaper. “But it brought both of us to the realization that we just work much better as friends . . . Now it’s a very mature situation.”

The friendship didn’t last long.

Two months later, Bieber popped the question in the Bahamas with a massive oval-shaped diamond ring. His Instagram post confirming the news has racked up more than 13 million likes.

“I’ve sat though a lot of Justin’s ex-girlfriends and whatnot, and I’m like, ‘Man, I don’t know.’ But with [Baldwin], I always knew,” JonBoy told The Post.

“They are the same person,” he continued. “They are just silly. They listen to the same music, they are into the same [clothing] brands. I just feel like they are meant for each other.”

JonBoy inked up the lovebirds one week before their engagement.

“Afterwards, Justin was like, ‘Let’s pray together,’ ” said JonBoy. “It was a beautiful moment.”

Still, he admitted: “I find myself, out of nowhere, praying for [Bieber and Baldwin] . . . I’m sure it’s not easy for them.”

Jonboy may not be the only insider praying for Bieber. Some individuals close to the singer are worried that he has rushed into marriage with Baldwin and is veering out of control.

According to TMZ, Bieber has also had a falling out with his mentor, Lentz. The pastor appeared surprised during a videotaped interview in Australia in June when the interviewer told him Bieber and Baldwin were engaged in July — offering a hesitant congratulations when prompted on camera.

Reached by phone on Thursday, the singer’s grandfather, George Bieber, said he also didn’t know his grandson was engaged or had gotten married.

“I wasn’t told,” said George. “He’s old enough, I guess. I was young when I got married, 17 or 18, but that didn’t last.”

Even Bieber’s team was caught off guard by the whirlwind romance, according to reports, with the duo forgoing a prenup, despite his reported $265 million fortune. (Baldwin is said to be worth a tidy $2 million herself.)

Things moved so fast, Bieber hasn’t yet had a chance to cover up the large Selena Gomez tattoo on his forearm, according to JonBoy.

“Sometimes that can be a reminder: ‘Hey, that’s not what I wanted to be. I’ve got someone way better for me than Selena,’ ” he said.

But who Bieber used to be also appears to still haunt him.

In a May 2016 Instagram post, the singer announced he would no longer take photographs with fans, because “I feel like a zoo animal.” He walked offstage in Manchester, England, five months later, after concertgoers refused to stop screaming.

Last May, Bieber begged fans on the street in Montclair, NJ, to “put down your phones and be normal for a second,” a newly common plea for a man seemingly rejecting the very attention that made him a superstar.

While Bieber once celebrated his stardom by living an outsize party life, more recently he’s resisting fame and all its trappings — and one insider who has worked with the pop star’s team thinks the world is now seeing the fallout of that confusion.

Said the source: “I think he has an ongoing identity crisis.”