diff --git a/site.hs b/site.hs
index 1a17d58..ab0af28 100644
--- a/site.hs
+++ b/site.hs
@@ -63,7 +63,11 @@ loadPage fp = do
   md <- io $ runIOorExplode $ readMarkdown markdownReadOpts (T.drop 5 markdown)
   {- find the main mount point for the page -}
   let mount =
-        T.unpack . just ("mount point of " ++ fp) $ yml ^? key "title" . _String
+        T.unpack . just ("mount point of " ++ fp) $ yml ^? key "mount" . _String
+  existing <- use $ pages . to (M.!? mount)
+  case existing of
+    Just pi -> error $ "mount for " ++ fp ++ " already exists from " ++ _pagePath pi
+    _ -> pure ()
   {- save to the state -}
   pages %=
     M.insert mount PageInfo {_pagePath = fp, _pageMeta = yml, _pageDoc = md}