I am trying to establish whether this is necessary to include in my program.

Sometimes, particularly when a tune is written in a key signature requiring several flats (or sharps), a double flat (or sharp) is required within the score.

I imagine that in theory, if a chord is based on this note, it would also have to have a double flat in its name.

Thus I suppose that, for example, Gbb7, or C##maj7 would be possible

Has anyone ever seen this in real scores? Would I have to allow for it when transposing? Or can I just ignore it?

What do other programs do?