The man known as “DD” was born with two penises.

We could dance around that singular (doubular?) fact, but there’s no use burying the anatomical anomaly that, overnight, made DD one of the Internet’s most viral celebrities.

Since birth, 25-year-old DD — who guards his real identity with militant care — has had a rare, congenital condition called diphallia, in which a duplicate penis develops during gestation. And since January 1, 2014, when DD gave Reddit one of its most popular Q&A sessions of all time, he has suffered another condition entirely: massive, uncontrolled Internet fame, the overwhelming, all-encroaching kind that has you abruptly ending long-term relationships, answering fan mail at all hours of the night, and charging $8 for a very brief, very graphic self-published autobiography — which DD has indeed recently done.

“When I ‘came out’ to the world on Reddit, I had no idea anyone would really care that much about me,” he writes in the book, punnily titled “Double Header.”

In conversation, he puts it a little more bluntly: Internet fame is “a double-edged sword,” he told The Post on Tuesday. “On one side, it’s been exciting and fun. On the other side it … causes me a great deal of anxiety from time to time.”

To understand why, one need only consider DD’s personal history — and the long, sad, exploitative legacy of so-called “human novelties” like his.

Diphallia is a rare condition; only 100 cases have been reported in the past 400 years. But in what little medical literature exists, diphallia is usually characterized as an unhappy thing: a herald of social ostracization, and spinal bifida, and a slate of reproductive complications too grotesque to explain here. DD has been relatively lucky — his complications are minimal, and his equipment’s in working order — but he didn’t exactly escape diphallia’s downsides.

He has, for starters, been treated to a life-long parade of prurient bystanders: gawking pediatricians, surprised friends and partners, unrelenting high-school bullies that made him despondent and suicidal, contemplating radical corrective surgery at 17.

He has experienced the occasional disgust, even horror, of would-be love interests and partners; at one point in “Double Header,” he describes an encounter in which a woman ran, screaming, from his room.

And yet, since DD’s story went viral, you’d be hard-pressed to find a guy more amped to put what is technically a congenital deformity on public display.

“[It’s] human nature to stare at something odd or gawk at something confusing, especially when it’s another person,” he explained. “On the flip side, it’s also human nature to either shy away from being gawked at and hide in the shadows, or courageously own the object of attention and display it proudly.”

To that end, DD has displayed his object(s) of attention on Tumblr — the site that started it all — as well as on Reddit and Twitter, where he has 20,000 followers. Just over a year ago, he agreed to do a community interview (or AMA) on Reddit, where users submitted more than 17,000 questions and brought DD to the attention of both the global media and a number of late-night talk show hosts. To date, his AMA is still Reddit’s fourth-most popular — Barack Obama, Bill Gates and David Attenborough.

Ranked in order, that’s president of the United States; world’s richest man; beloved naturalist; guy with two penises. (There can be no better summation of the Internet’s varied interests.)

DD’s fame isn’t particularly surprising, of course: America has a long, gross, gawking affinity for “human novelties,” whether they appear in carnival sideshows or on reality TV. Long, long before there was a guy with extra junk, there were dwarf children who smoked cigars and conjoined twins who hobnobbed with royals, for fun or for profit.

It’s impossible not to see something of the 19th-century freak show in DD’s shtick; even on the relatively quiet, friendly Tumblr blog where his fame started, there’s a lot of unabashed ogling, a lot of deeply personal questions. And while his particular claim to Internet fame makes the freak-show parallel slightly more literal, it’s an analogy that applies pretty well to most “human memes” — people reduced to one unusual trait or photograph or quote, and then passed around by the salivating Internet masses, with or without their permission. I’m reminded of Alex from Target, the ordinary high-schooler who became something of an underage sex icon when teen girl fandoms discovered his picture. Or even “Bad Luck Brian,” profiled by my colleague Jessica Contrera yesterday; the real Brian’s name isn’t even Brian, and he’s spent a long time extricating himself from the pressures and expectations of online celebrity.

Celebrity, it turns out, isn’t always so great when it’s unasked for. In fact, it sometimes looks a whole lot like exploitation.

DD doesn’t quite see it that way, of course, which is both what makes his story interesting to follow and what separates him from the novelties of yore. He’s been militant about controlling his own narrative: he won’t post pictures of his face, he won’t reveal any even vaguely personal details, and he absolutely won’t do porn. (“Not interested in it,” he told one interviewer early on. “[Even] the fact that I’m sending you pix is really weird for me. People think of it as an anomaly but it’s my body.”) He runs his own accounts on Tumblr and Twitter, where he loves getting messages from followers and fans. And he isn’t making any money off the ebook, he claims: that’s all going to an unnamed publisher. He just wanted to get out his story in “his own words,” to transcend the one-dimensionality of DD as meme.

He’s okay with the people who see him as some kind of sex object. (“The gawking amuses me,” he says. “You have to realize, I’ve looked down at these boys all my life.”) But he’s also interested in something a little bigger: If he can parlay diphallia into something like fame and happiness, then every bullied kid who feels “different” has to have some hope of overcoming or owning their differences, too. DD corresponds regularly with people in that category, he says; he actually used to aim uplifting Twitter messages directly to them.

“We may all be different looking,” he explained, “but inside we’re all human.”

It’s not exactly a profound sentiment, sure, but it’s definitely an empowered one — particularly coming from a guy whose fame is based entirely on the fact that he looks different. If the Internet is the freak show of the 21st century, at least we’re freaking out with a little more humanity.