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It all started with a case of mistaken identity at a Las Vegas hotel. Singer Jason Derulo swung at two fans who mistook him for another R&B singer — Usher.

Police sources tell TMZ cops were called to the ARIA hotel early Tuesday morning after Derulo threw punches at the men.

Eyewitnesses say Jason and one of the men were passing each other on an escalator when the guy shouted, “Hey, Usher! F*** you, bitch!!!”

Jason apparently took offense to the disrespect and lunged at the guy. He punched the guy in the face, sending him sprawling to the floor.

Jason allegedly slapped the second man just for breathing. Security stepped in to break up the fracas. Video shows Jason pushed his burly bodyguard aside to get at the guys.

Apparently, Jason had gotten into it earlier with the same two guys for calling him “Usher.”

At press time, the 2 victims declined to press charges, but there’s always the possibility of a lawsuit, says TMZ. Jason was not arrested, but he did get a trespassing notice from ARIA Hotel, and was promptly thrown off the property.