diff --git a/lib/lists.nix b/lib/lists.nix
index 927358af7c1..ed1b7e74399 100644
--- a/lib/lists.nix
+++ b/lib/lists.nix
@@ -625,13 +625,15 @@ rec {
        => [ 3 2 4 ]
    */
   unique = list:
-    if list == [] then
-      []
-    else
-      let
-        x = head list;
-        xs = unique (drop 1 list);
-      in [x] ++ remove x xs;
+    let isFirst = value: end:
+      if end == 0 then true else
+      if elemAt list (end - 1) == value then false
+      else isFirst value (end - 1);
+    in concatMap (index:
+      let value = elemAt list index;
+          shouldInclude = isFirst value index;
+      in optional shouldInclude value
+    ) (range 0 (length list - 1));
 
   /* Intersects list 'e' and another list. O(nm) complexity.