I often run into this frustrating problem when using layer masks.
I have a layer to which I have added a layer mask. Now I select a black brush and paint over it with 90% opacity to hide that part of the layer and reveal the layer below which is grey. If I now paint again over the same area using the same brush, the effect sort of adds up and I now see that area as black instead of grey.
Is there any way at all that I can avoid this? Is there any setting with which the brush will not paint the part that I have already painted?