diff --git a/nixos.nix b/nixos.nix
index 307355b..9add1a0 100644
--- a/nixos.nix
+++ b/nixos.nix
@@ -9,6 +9,7 @@ packages:
 let
   inherit (lib)
     lists
+    attrsets
     mkMerge
     mkIf
     mapAttrs
@@ -238,15 +239,22 @@ in
             ++ foldlAttrs (
               acc': _: app:
               acc'
-              ++ [
-                {
-                  ${getsubname fid app.identity} = mkMerge [
-                    cfg.extraHomeConfig
-                    app.extraConfig
-                    { home.packages = app.packages; }
-                  ];
-                }
-              ]
+              ++ (
+                let
+                  key = getsubname fid app.identity;
+                in
+                [
+                  {
+                    ${key} = mkMerge (
+                      [
+                        cfg.extraHomeConfig
+                        { home.packages = app.packages; }
+                      ]
+                      ++ (if (attrsets.hasAttrByPath [ key ] acc) then (if !app.shareUid then throw "unexpected username collision" else [ ]) else [ app.extraConfig ])
+                    );
+                  }
+                ]
+              )
             ) [ { ${getsubname fid 0} = cfg.extraHomeConfig; } ] cfg.apps
           ) [ privPackages ] cfg.users
         );
diff --git a/test/configuration.nix b/test/configuration.nix
index 0bee0b2..f530cb7 100644
--- a/test/configuration.nix
+++ b/test/configuration.nix
@@ -96,13 +96,19 @@
     users.alice = 0;
 
     extraHomeConfig = {
-      home.stateVersion = "23.05";
+      # To test merge deduplication:
+      options._fortify.noop = lib.mkEnableOption "nothing";
+
+      config = {
+        home.stateVersion = "23.05";
+      };
     };
 
     apps = {
       "cat.gensokyo.extern.foot.noEnablements" = {
         name = "ne-foot";
         identity = 1;
+        shareUid = true;
         verbose = true;
         share = pkgs.foot;
         packages = with pkgs; [
@@ -130,7 +136,8 @@
 
       "cat.gensokyo.extern.Alacritty.x11" = {
         name = "x11-alacritty";
-        identity = 3;
+        identity = 1;
+        shareUid = true;
         verbose = true;
         share = pkgs.alacritty;
         packages = with pkgs; [
diff --git a/test/test.py b/test/test.py
index b912d6e..4b67b6f 100644
--- a/test/test.py
+++ b/test/test.py
@@ -206,9 +206,9 @@ machine.wait_until_fails("pgrep foot", timeout=5)
 
 # Test XWayland (foot does not support X):
 swaymsg("exec x11-alacritty")
-wait_for_window(f"u0_a{aid(2)}@machine")
+wait_for_window(f"u0_a{aid(0)}@machine")
 machine.send_chars("clear; glinfo && touch /tmp/x11-ok\n")
-machine.wait_for_file(tmpdir_path(2, "x11-ok"), timeout=15)
+machine.wait_for_file(tmpdir_path(0, "x11-ok"), timeout=15)
 collect_state_ui("alacritty_x11")
 check_state("x11-alacritty", 2)
 machine.send_chars("exit\n")