Mark Wahlberg Surprises Crowd at New Kids on the Block Concert!

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It was double the Wahlbergs at New York City's Madison Square Garden on Monday night!

Mark Wahlberg -- formerly known as Marky Mark -- surprised thousands of fans at the New Kids on the Block show when he showed up during his brother Donnie Wahlberg's performance of "Good Vibrations."

This was the first time the Ted 2 star has joined his brother's boy band on stage in over 20 years.

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And he wasn't the only surprise star of the night! Rapper 50 Cent introduced Mark to the excited audience, exclaiming that it looked like there were some "good vibrations here tonight."

The 39-year-old hip-hop star was also joined by Biz Markie. The rappers performed their hits "Just a Friend" and "In Da Club."

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's Main Event tour has been thrilling fans across the country since they first kicked off their 40-concert series in Las Vegas on May 1. They were also joined by '90s chart-toppers TLC and rapper Nelly.

ET was there during their tour rehearsals. Check it out: