AMNA NAWAZ: Among the many fairs and festivals this summer, there's only one where seeing double doesn't require a drink.

The Twins Days Festival bills itself as the largest annual gathering of twins in the world.

While the festival celebrates the unique bond between twins, it's also a hub for scientists studying what makes them so alike and so different.

Deema Zein doubles the fun in Twinsburg, Ohio.

DEEMA ZEIN: Thirty-six-year-old Vilma and Cesia Gutierrez love being identical twins.

CESIA GUTIERREZ, Twin: My favorite part of being a twin is just knowing that I always have her, and like I can always say it's two against the world.

DEEMA ZEIN: Years ago, they stumbled upon a festival that made them do a double-take, the Twins Days Festival in Twinsburg, Ohio.

MAN: Happy Twins Day.

DEEMA ZEIN: For the last 50 years, twins and multiples from across the globe meet every August to celebrate their twinness.

ADAM CORDNER, Twin: Coming here just kind of reminds you how cool it is to be a twin and special.

And... NATHAN CORDNER, Twin: You are special.

ADAM CORDNER: I am special.

TERRI COLEMAN-RANDOLPH, Twin: We're in a twin bubble all weekend.

And people aren't asking you silly twin questions.

JAMES WILSON, Twin: You feel normal because no one's looking, like, are you twins?

KAITLYN HAWTHORNE, Twin: There's just no other kind of bond than being a twin.

DEEMA ZEIN: A bond that has its struggles.

TRACY RENBARGER, Twin: What about our fights?

TERRI JELINEK, Twin: So I... TRACY RENBARGER: We fight like no other.

NATHAN CORDNER: And the worst part, I look like him.

(LAUGHTER) DEEMA ZEIN: But also major perks.

JAMIS KUKURA, Twin: We're in the same grade.

So we can just help each other on homework and stuff.

HEIDI RYALL, Twin: She couldn't finish her class.

She got pregnant and so I finished it for her.

I got her from a D to B, and the teacher never caught on.

HANNAH WOLFE, Twin: You're not supposed to say what my grade was.

DEEMA ZEIN: And even a vocabulary of its own.

MELISSA SMITH, Twin: She's A and I'm B. MELANIE HERREN, Twin: A is just this the twin that was born first, and B is the twin that was born second.

It's just twin lingo that you catch on to.

DEEMA ZEIN: The twin bond isn't just a theme of the festival.

It's baked into the identity of the city that hosts it, says executive director Andy Miller.

ANDY MILLER, Executive Director, Twins Days Festival: There's a lot of wonderful fairs and festivals throughout Ohio and the United States.

I think one of the things that makes this special is the subject of our event are people, and they just happen to be identical twins.

DEEMA ZEIN: Twinsburg was once called Millsville until it was renamed after a pair of identical twins in the early 19th century.

ANDY MILLER: Aaron Wilcox and Moses Wilcox were landowners, and they donated six acres of property, which now is the town square, along with $20 towards the first schoolhouse in exchange for renaming Millsville to Twinsburg in honor of their twinship.

DEEMA ZEIN: The festival began in 1976, when 36 sets of twins gathered to celebrate the city's twin founders.

Today, the festival welcomes around 2,000 sets of twins each year.

ANDY MILLER: This year, we decided to have an 80s theme, and it's called Twins Day's Totally Two-Bular with T-W-O.

JAQUADA ROYAL-CROCKETT, Twin: We're paying homage to the late Tina Turner.

LAQUALA ROYAL-BARTEK, Twin: Legendary.

DEEMA ZEIN: Along with a parade and contests, ranking twins on everything from most alike and least alike to youngest and oldest... MAN: Ninety and one-half.

DEEMA ZEIN: ... there's one surprising benefit, unique opportunities for scientific research.

DR.

KELLY SCARBERRY, University Hospitals: With identical twins, they share the exact same DNA.

So we're able to specifically study whether environmental factors or DNA contribute to them developing diseases.

DEEMA ZEIN: Dr.

Kelly Scarberry is a dermatologist at University Hospitals among the many research institutions set up at the festival.

At his tent, twins scan a Q.R.

code and fill out a survey.

DR.

KELLY SCARBERRY: Asking about whether they have certain conditions, skin diseases, and determining whether any environmental factors they have come across in their life is contributing to these diseases.

ANDY MILLER: They can gather enough data in a day-and-a-half up here at the festival that would normally take them about three years to gather that information, because it's all contained here and there's a plethora of subjects to test from.

DEEMA ZEIN: Researchers from the FBI to beauty brands have used Twin Festival participants to study skin aging, burnout, and facial recognition technology.

SKYLER NICK, Twin: Because I'm able to unlock his iPhone.

SPENCER NICK, Twin: And I'm able to unlock his iPhone.

So they haven't gotten it down pat yet.

DEEMA ZEIN: But for the twins, many of whom return year after year, it's the relationships they form that keep them coming back.

DONALD FITZGERALD, Twin: Every year we come back, we keep meeting more new friends.

VILMA GUTIERREZ, Twin: This has honestly become like our second family.

CESIA GUTIERREZ: Yes.

VILMA GUTIERREZ: And, fortunately, last year, I met my husband, so I was able to marry a twin.

We have a baby now.

DEEMA ZEIN: While Vilma Gutierrez's baby girl is what twins call a singleton, without a twin of her own, she's still surrounded by double the love.

For the "PBS News Hour," I'm Deema Zein.