RenderDoc supports a pixel history view, showing the list of all modification events that happened to a specified pixel. To launch it, simply pick the pixel you would like to view the history of in the texture viewer, and click the 'history' button underneath the zoomed-in pixel context.
Each event will show up red or green depending on whether it affected or didn't affect the pixel. By expanding the event, you can see the possibly several primitives within the draw that overdrew the pixel.