Here's a simple test.
Open an RGB image, flatten, desaturate, such that you have one grayscale layer in RGB.
Add a blank layer above.
Above that add a layer in color mode, fill with color and set the opacity to 50%.
Spot Heal on the blank layer.
You will see that the spot healing contains color from above and the effect of the color fill will be "doubled" in those areas.
A fill layer in color mode isn't an adjustment layer. I don't know why it makes a difference, but it does. Try this:
- blank layer above image layer
- hue/sat adjustment layer above the blank layer
In the hue/sat dialog, tick "Colorize" and crank up the saturation. Then spot heal on the blank layer. No doubling of the color effect. It still seems safe to say that if you spot heal on a blank layer below
adjustment layers, you won't get any doubling effect.