Cancelling a double sharp/flat all the way to natural uses a single natural sign, never two

Fully cancelling a double accidental to the unaltered pitch uses ONE natural sign, never two.

A double-flat Bbb cancels to plain B with two natural glyphs instead of one.

Gould; Read; Ross -- universal agreement, unlike the partial-cancellation case above.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.