Extracting Identical Pixels Between Two Layers

[Ofnuts]

1. Align the layers
   
2. Put the top one in "Difference" mode: identical pixels in both layers will yield a perfectly black result.
   
3. Do Layer > New from visible to snapshot this into a  new layer
   
4. Use the Threshold tool to set to white any pixels that is not perfectly black, to obtain an image where all white pixels correspond to different pixels in the compared images
   
5. If you want a selection from this, open the Channels list, right click on any R/G/B channel and "Channel to selection". You can then hide/discard the Visible layer.