I am sorry that you find this behavior weird. I can only guess what you want to happen. If you set the opacity of the brush to 100% this will not happen. If you instead set the foreground color to 50% gray or whatever you want it to be then paint the overlap will still be 50% gray. If you set the brush opacity to 50% then when you overlap the result will be 50% gray plus 50% of 50% or 75%. That is what it should do.