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Sean Kaufman must be team Conrad ... 'cause he just planted a smooch on his "The Summer I Turned Pretty" costar Christopher Briney !

Check it out -- the pair were spotted at the New York Knicks game Friday evening at NYC's Madison Square Garden and when the camera was on them ... Sean went in for a big one! Getty

The actors put on a show for plenty of fans and celebs in attendance, which included Ray Romano , Jordyn Woods and Spike Lee ... see them in our gallery below. Gallery Launch Gallery Getty

The Knicks picked up a W against the Milwaukee Bucks, advancing them to the knockout round of the NBA Cup.

We're sure there will be plenty more courtside celeb sightings as the stakes get higher -- stay tuned!

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