Ensure anchor links are unique per document (#574)

lib/core/toSlug.js

@@ -18,7 +18,16 @@ const exceptAlphanum = new RegExp(
   'g'
 );
 
-module.exports = string => {
+/**
+ * Converts a string to a slug, that can be used in heading anchors
+ *
+ * @param  {string} string
+ * @param  {Object} [context={}] - an optional context to track used slugs and
+ *                                 ensure that new slug will be unique
+ *
+ * @return {string}
+ */
+module.exports = (string, context = {}) => {
   //  var accents = "àáäâèéëêìíïîòóöôùúüûñç";
   const accents =
     '\u00e0\u00e1\u00e4\u00e2\u00e8' +
@@ -50,5 +59,22 @@ module.exports = string => {
     slug += '-';
   }
 
+  if (!context.slugStats) {
+    context.slugStats = {};
+  }
+
+  if (typeof context.slugStats[slug] === 'number') {
+    // search for an index, that will not clash with an existing headings
+    while (
+      typeof context.slugStats[slug + '-' + ++context.slugStats[slug]] ===
+      'number'
+    );
+    slug += '-' + context.slugStats[slug];
+  }
+
+  // we are tracking both original anchors and suffixed to avoid future name
+  // clashing with headings with numbers e.g. `#Foo 1` may clash with the second `#Foo`
+  context.slugStats[slug] = 0;
+
   return slug;
 };