diff --git a/nixos/modules/system/boot/systemd-lib.nix b/nixos/modules/system/boot/systemd-lib.nix
index a33602915867e..fa109394fedbe 100644
--- a/nixos/modules/system/boot/systemd-lib.nix
+++ b/nixos/modules/system/boot/systemd-lib.nix
@@ -114,7 +114,9 @@ in rec {
         (if isList value then value else [value]))
         as));
 
-  generateUnits = type: units: upstreamUnits: upstreamWants:
+  generateUnits = generateUnits' true;
+
+  generateUnits' = allowCollisions: type: units: upstreamUnits: upstreamWants:
     pkgs.runCommand "${type}-units"
       { preferLocalBuild = true;
         allowSubstitutes = false;
@@ -182,8 +184,13 @@ in rec {
           if [ "$(readlink -f $i/$fn)" = /dev/null ]; then
             ln -sfn /dev/null $out/$fn
           else
-            mkdir -p $out/$fn.d
-            ln -s $i/$fn $out/$fn.d/overrides.conf
+            ${if allowCollisions then ''
+              mkdir -p $out/$fn.d
+              ln -s $i/$fn $out/$fn.d/overrides.conf
+            '' else ''
+              echo "Found multiple derivations configuring $fn!"
+              exit 1
+            ''}
           fi
        else
           ln -fs $i/$fn $out/
diff --git a/nixos/modules/system/boot/systemd-nspawn.nix b/nixos/modules/system/boot/systemd-nspawn.nix
index 1e2435e36f0c1..06ea5ee49f727 100644
--- a/nixos/modules/system/boot/systemd-nspawn.nix
+++ b/nixos/modules/system/boot/systemd-nspawn.nix
@@ -116,7 +116,7 @@ in {
     in 
       mkMerge [
         (mkIf (cfg != {}) { 
-          environment.etc."systemd/nspawn".source = mkIf (cfg != {}) (generateUnits "nspawn" units [] []);
+          environment.etc."systemd/nspawn".source = mkIf (cfg != {}) (generateUnits' false "nspawn" units [] []);
         })
         {
           systemd.targets.multi-user.wants = [ "machines.target" ];