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Lindsay Lohan got a surprise visit while she was ensconced in rehab -- we found out Ben Affleck had a secret meeting with her as a show of support.

Affleck -- who himself was in rehab back in 2001 -- met Lindsay somewhere away from the Cliffside rehab facility in Malibu where Lindsay was getting treatment. We're told Ben gave her guidance on how to maintain sobriety after rehab for someone in Hollywood.

We're told Ben talked to Lindsay about how to make a comeback in Hollywood -- a subject on which he's an expert. Ben talked to Lindsay about her career -- on how talented she is and how she has the ability to turn things around.

Ben also told her how supportive the entertainment community is for people who make a real effort to get better and stay clean. Our sources say the meeting really touched her.