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Presidential candidate Julian Castro is often mistaken for his identical twin brother, Joaquin, but now there’s a whisker of a difference between the two.

The 44-year-old siblings travel together on the campaign trail, where Julian, a former federal housing secretary mayor of San Antonio, Texas, is running for the Democratic nomination with the help of top adviser Joaquin, a US representative.

Reporters were thrown off by the pair at last month’s Democratic presidential debate in Miami, when the brothers entered the media room afterward in similar suits.

And at a candidates’ forum in April, organizers mistakenly used a photo of Joaquin.

Julian took it in stride, saying it was OK because his brother is “better-looking.”

But Joaquin has come up with a solution to the hairy situation: He’s growing a beard.

“I hadn’t shaved in, like, five days, and I thought I’ll just go ahead and grow it, you know?” Joaquin, who reps the San Antonio area, told The Post.