When I teach my classes I tell them that there’s no such thing as a diminished unison (or “förminskad prim”, as we call it in Swedish). A perfect unison is the same note, and you can’t decrease the distance between it, only increase it.
But in Dorico I find that the term “diminished unison” is used to transpose a note half a step downwards. It should properly be called transposing a note an augmented unison downwards.
Or are there differing traditions regarding unisons?