diff --git a/infer/src/istd/IList.ml b/infer/src/istd/IList.ml
index 48fc0b502..a3a370a58 100644
--- a/infer/src/istd/IList.ml
+++ b/infer/src/istd/IList.ml
@@ -90,7 +90,7 @@ let rec merge_sorted_nodup ~cmp ~res xs1 xs2 =
 
 
 let inter ~cmp xs ys =
-  let rev_sort xs = List.sort ~compare:(fun x y -> cmp y x) xs in
+  let rev_sort xs = List.dedup_and_sort ~compare:(fun x y -> cmp y x) xs in
   let rev_xs = rev_sort xs in
   let rev_ys = rev_sort ys in
   let rec inter_ is rev_xxs rev_yys =
@@ -100,7 +100,7 @@ let inter ~cmp xs ys =
     | x :: rev_xs, y :: rev_ys ->
         let c = cmp x y in
         if Int.equal c 0 then inter_ (x :: is) rev_xs rev_ys
-        else if c < 0 then inter_ is rev_xs rev_yys
+        else if c > 0 then inter_ is rev_xs rev_yys
         else inter_ is rev_xxs rev_ys
   in
   inter_ [] rev_xs rev_ys
diff --git a/infer/src/unit/IListTests.ml b/infer/src/unit/IListTests.ml
new file mode 100644
index 000000000..1a4ca85a5
--- /dev/null
+++ b/infer/src/unit/IListTests.ml
@@ -0,0 +1,39 @@
+(*
+ * Copyright (c) 2018-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *)
+
+open! IStd
+open OUnit2
+
+let inputs =
+  [ ("empty", [])
+  ; ("0", [0])
+  ; ("0s", [0; 0; 0])
+  ; ("1", [1])
+  ; ("1s", [1; 1])
+  ; ("1_2", [1; 2])
+  ; ("1_2_3", [1; 2; 3])
+  ; ("1_3_2s", [1; 3; 2; 2; 2; 2])
+  ; ("3_1_2", [3; 1; 2])
+  ; ("0_3_2", [0; 3; 2])
+  ; ("3s_1s", [3; 1; 3; 1; 3])
+  ; ("4", [4]) ]
+
+
+let tests =
+  let inter_test input1 input2 _ =
+    let using_list = IList.inter ~cmp:Int.compare input1 input2 in
+    let using_set =
+      IntSet.inter (IntSet.of_list input1) (IntSet.of_list input2) |> IntSet.elements
+    in
+    assert_equal using_list using_set
+  in
+  let tests_ =
+    List.concat_map inputs ~f:(fun (name1, input1) ->
+        List.map inputs ~f:(fun (name2, input2) ->
+            "inter_" ^ name1 ^ "_with_" ^ name2 >:: inter_test input1 input2 ) )
+  in
+  "IList_tests" >::: tests_
diff --git a/infer/src/unit/inferunit.ml b/infer/src/unit/inferunit.ml
index a2f55528b..585f221b6 100644
--- a/infer/src/unit/inferunit.ml
+++ b/infer/src/unit/inferunit.ml
@@ -34,6 +34,7 @@ let () =
         ; DifferentialTests.tests
         ; DifferentialFiltersTests.tests
         ; FileDiffTests.tests
+        ; IListTests.tests
         ; JavaProfilerSamplesTest.tests
         ; PerfProfilerATDParserTest.tests
         ; ProcCfgTests.tests