clash detection

Question

I conncected two walls with L connection , anybody have idea why it detected as clash in clash detection 
 see attachment

Steve Cocchi · Answer

OK, they do appear to be identical. 
 I would first suggest using the Connect Forms As L (Bisector) tool on those two walls, just to be sure that the joint does not contain any overlapping geometry. Based on the image alone that's the only thing I can think of.