diff --git a/sorted_map/map.mbt b/sorted_map/map.mbt
index 09197c0e8..fb2766f43 100644
--- a/sorted_map/map.mbt
+++ b/sorted_map/map.mbt
@@ -40,7 +40,12 @@ pub fn of[K : Compare, V](entries : Array[(K, V)]) -> T[K, V] {
 pub fn add[K : Compare, V](self : T[K, V], key : K, value : V) -> Unit {
   let parent = loop self.root, self.root {
     Some(curr), _ =>
-      continue if key < curr.key { curr.left } else { curr.right }, Some(curr)
+      if key == curr.key {
+        curr.value = value
+        return
+      } else {
+        continue if key < curr.key { curr.left } else { curr.right }, Some(curr)
+      }
     None, parent => parent
   }
   let new = new_node(key, value, ~parent)
@@ -51,7 +56,6 @@ pub fn add[K : Compare, V](self : T[K, V], key : K, value : V) -> Unit {
   } else {
     parent.unwrap().right = Some(new)
   }
-  new.parent = parent
   self.balance(new)
   self.size += 1
 }
diff --git a/sorted_map/map_test.mbt b/sorted_map/map_test.mbt
index 6dddaa1f0..c36f98695 100644
--- a/sorted_map/map_test.mbt
+++ b/sorted_map/map_test.mbt
@@ -41,6 +41,13 @@ test "add" {
   inspect(map.size(), content="8")?
 }
 
+test "add duplicate key" {
+  let map = of([(3, "c"), (2, "b"), (1, "a")])
+  inspect(map.debug_tree(), content="([1]2,b,([0]1,a,_,_),([0]3,c,_,_))")?
+  map.add(1, "x")
+  inspect(map.debug_tree(), content="([1]2,b,([0]1,x,_,_),([0]3,c,_,_))")?
+}
+
 test "RL rotate" {
   let map = new()
   map.add(5, "a")