Sprites/Diffuse shader overlap problem

Hi everybody,

I'm making a game with a 2D Zelda camera, so I use the Sorting Layer system in the sprite render, and the Order in Layer value is set with the current Y position value. But it happens sometimes that 2 characters are in the same Y position, then have the same Order in Layer. With the Sprites/Default shader there is no problem but with the Sprites/Diffuse one it appears that it applies a multiply on the 2 sprites, so I can see the second behind the first when they are close to a Point Light.

Can someone help me to fix it ? I don't want to have a script which check if 2 characters are in the same Y axis...