Name the position a directive name the other one spells belongs to

src/spec.test.ts

@@ -8,6 +8,7 @@ import type { AttributeKind, AttributeVocabulary } from './adf/attribute-vocabul
 import { blockDirectives } from './adf/block-directives.ts'
 import { inlineDirectives } from './adf/inline-directives.ts'
 import { markAttributes } from './adf/mark-attributes.ts'
+import { textDirectiveName } from './markdown/text-directive.ts'
 
 type Declared = { attributes: AttributeVocabulary }
 
@@ -96,6 +97,12 @@ test('the inline node table holds the attributes spec/flavour.md gives each node
   assert.deepEqual(declarations('Inline nodes'), vocabularies(inlineDirectives))
 })
 
+// A name in two tables would make the position a directive is read in ambiguous.
+test('no name is spelled in more than one position', () => {
+  const names = [...Object.keys(blockDirectives), ...Object.keys(inlineDirectives), ...Object.keys(markAttributes), textDirectiveName]
+  assert.equal(new Set(names).size, names.length)
+})
+
 test('the mark table holds the attributes spec/flavour.md gives each mark', () => {
   assert.deepEqual(declarations('Marks'), vocabularies(Object.fromEntries(Object.entries(markAttributes).map(([type, attributes]) => [type, { attributes }]))))
 })