diff --git a/nix/lib/config/options.nix b/nix/lib/config/options.nix
index 8387ec3..0ced1a9 100644
--- a/nix/lib/config/options.nix
+++ b/nix/lib/config/options.nix
@@ -600,5 +600,25 @@ in
     in map (addr: {
       assertion = builtins.length addrHosts.${addr} == 1;
       message = "Address ${addr} is assigned to more than one host: ${lib.concatStringsSep " " addrHosts.${addr}}";
-    }) (builtins.attrNames addrHosts));
+    }) (builtins.attrNames addrHosts))
+    ++
+    # duplicate vlan check
+    (let
+      vlanNets =
+        builtins.foldl' (result: net:
+          let
+            vlan = toString config.site.net.${net}.vlan;
+          in
+            if result ? ${vlan}
+            then result // {
+              "${vlan}" = result.${vlan} ++ [ net ];
+            }
+            else result // {
+              "${vlan}" = [ net ];
+            }
+        ) {} (builtins.attrNames config.site.net);
+    in map (vlan: {
+      assertion = builtins.length vlanNets.${vlan} == 1;
+      message = "VLAN ${vlan} is used by more than one network: ${lib.concatStringsSep " " vlanNets.${vlan}}";
+    }) (builtins.attrNames vlanNets));
 }