Single slabs can be made into double slabs when aiming at the sides of blocks, even if said half of said block corresponds to the half of the block the existing placed slab occupies. This is usually impossible to encounter with full blocks but those with nonstandard shapes make it easy to reproduce. This is inconsistent with the placement of single slabs.

1. Create the following block arrangement:

2. Aim at this part of the stair:

3. Right click to place, and note that the slab has been placed in the top half of the adjacent block despite there not being a bottom half for that slab to occupy nor were you aiming at the top half of the block.

Single slab placement behaviour for comparison: