Rihanna -- Trespasser Arrested -- He May Have Been There Before

2:12 PM PT: We've learned cops now say the trespasser and the attempted burglar are one in the same so they've just booked him on the burglary charge. Law enforcement sources tell us he's a 27-year-old man who believes he and Rihanna are friends.

Lesson 1: NEVER return to the scene of the crime. Cops think that's exactly what a rather dumb dude did this morning at Rihanna's house ... a dumb dude who is now under arrest.

The guy was trying to scale the singer's wall at her Pacific Palisades home at around 11 am. Rihanna's security called 911 and cops came out and arrested the trespasser ... they found him about a mile from her home.

Now here's the dumb part. Someone trespassed on Rihanna's property on Sunday and threw a chair through her window. Cops are investigating the attempted burglary, and our law enforcement sources say they've reviewed the security footage and the would-be burglar looks VERY MUCH like the guy who was arrested today.

Rihanna was not home during today's incident.