Add guardrail against duplicate slugs

lib/DB.rakumod

@@ -71,6 +71,12 @@ class PostDB {
 
    #| Render the site to the provided output directory
    method render(IO::Path:D $out-dir, Config:D :$config = Config.new) {
+       ## Consistency checks
+       # Check to make sure all the slugs are unique
+       my @all-the-slugs = %!posts.values.map(*.all-slugs).flat;
+       die "Duplicate slug detected"
+           unless @all-the-slugs.unique.elems == @all-the-slugs.elems;
+       ## Rendering
        my $posts = $out-dir.add('posts/');
        my $by-id = $posts.add('by-id/');
        my $by-slug = $posts.add('by-slug/');

lib/DB/Post.rakumod

@@ -63,7 +63,8 @@ method all-slugs(--> Array[Str:D]) {
     my Str:D @slugs = @!slugs.clone;
     @slugs.push($long-title-slug);
     @slugs.push($six-word-slug);
-    @slugs;
+    @slugs.=unique;
+    @slugs
 }
 
 #| Render this post to an html body