When placing two slabs on top of each other during the planning stage, thus creating a double slab block, during the commit phase, the planner expects a double slab block in the supply chest to complete that block, instead of two single slabs. Double slabs are unobtainable without using admin commands.

Cool, thanks - I have fixed this now in 0.4.0 :) It will still report needing 2 double slabs though - i thought for clarity.. but perhaps I should actually just say 2 slabs since it is the materials required?

I'll mark that for the next release