diff --git a/overlay.nix b/overlay.nix
index 122729d..c3aab71 100644
--- a/overlay.nix
+++ b/overlay.nix
@@ -10,9 +10,19 @@ let
   nameValuePair = n: v: { name = n; value = v; };
   nurAttrs = import ./default.nix { pkgs = super; };
 
+  overlay =
+    builtins.listToAttrs
+      (map (n: nameValuePair n nurAttrs.${n})
+        (builtins.filter (n: !isReserved n)
+          (builtins.attrNames nurAttrs)));
+
 in
 
-  builtins.listToAttrs
-  (map (n: nameValuePair n nurAttrs.${n})
-  (builtins.filter (n: !isReserved n)
-  (builtins.attrNames nurAttrs)))
+# Shadow existing package sets if they already exist rather than replacing.
+overlay //
+{
+  goModules = (super.goModules or { }) // (overlay.goModules or { });
+  jetbrains = (super.jetbrains or { }) // (overlay.jetbrains or { });
+  python2Packages = (super.python2Packages or { }) // (overlay.python2Packages or { });
+  python3Packages = (super.python3Packages or { }) // (overlay.python3Packages or { });
+}