diff --git a/src/manifold/deferred.clj b/src/manifold/deferred.clj
index 80f87f00..7c4a31ac 100644
--- a/src/manifold/deferred.clj
+++ b/src/manifold/deferred.clj
@@ -1290,6 +1290,7 @@
   (let [[_ bindings & body] (walk/macroexpand-all `(let ~bindings ~@body))
         locals              (keys (compiler/locals))
         vars                (->> bindings (partition 2) (map first))
+        _                   (assert (apply distinct? vars) "Let binding vars must be unique.")
         marker              (gensym)
         vars'               (->> vars (concat locals) (map #(vary-meta % assoc marker true)))
         gensyms             (repeatedly (count vars') gensym)