Q. How to Mesh Unwrap Painting overwrite?

I do not use English well. I’m sorry.

I am working on Mesh Unwrap Painting project.
I have a blockage now.

I paint the mesh and save it as a RenderTarget Texture.
But R,G,B Channel Value is Combined.

i don`t combined Channel, just need overwrite.
I want to be overwritten with the last one drawn.



I want to do like picture on the left, not the picture on the right.
Anybody Help me.