Fix repeated slugs

Fixes graphql#12

Author: leebyron

site/_core/Header.js

@@ -38,7 +38,14 @@ export function toSlug(string) {
 }
 
 export default (props) => {
-  var slug = toSlug(props.toSlug || props.children);
+  var usedSlugs = props.usedSlugs || {};
+  var append = '';
+  var loopCount = 0;
+  do {
+    var slug = toSlug((props.toSlug || props.children) + append);
+    append = '-' + (++loopCount);
+  } while (usedSlugs[slug]);
+  usedSlugs[slug] = slug;
   var Heading = 'h' + props.level;
   var url = props.url || '';
 

site/_core/Marked.js

@@ -745,6 +745,7 @@ function Parser(options) {
   this.tokens = [];
   this.token = null;
   this.options = options || marked.defaults;
+  this.usedSlugs = {};
 }
 
 /**
@@ -816,7 +817,7 @@ Parser.prototype.tok = function() {
     }
     case 'heading': {
       return (
-        <Header url={this.options.url} level={this.token.depth} toSlug={this.token.text}>
+        <Header url={this.options.url} level={this.token.depth} toSlug={this.token.text} usedSlugs={this.usedSlugs}>
           {this.inline.output(this.token.text)}
         </Header>
       );