In this case blender confused me.
I add cube -> select it (object mode)-> scale it set Y=0,
Now it becomes a plane. -> go to Edit mode and select vertice of one conner
Remove double, it showed “removed 0 vertices”
But if I merge it, blender told me “removed 1 vertices”
If I scaled cube to plane in Edit mode, “remove double” and “merge” gave same result of “removed 1 vertices”