diff --git a/home-manager/default.nix b/home-manager/default.nix
index 144fd7f..90f1e64 100644
--- a/home-manager/default.nix
+++ b/home-manager/default.nix
@@ -2,7 +2,7 @@
 let
   inherit (builtins) head tail mapAttrs attrValues;
   inherit (lib) flatten optional mkOption mkDefault mkOptionType;
-  inherit (lib.my) homeStateVersion mkOpt' commonOpts inlineModule';
+  inherit (lib.my) homeStateVersion mkOpt' commonOpts inlineModule' applyAssertions;
 
   cfg = config.home-manager;
 
@@ -40,7 +40,11 @@ let
     ] ++ (tail defs);
   };
 
-  homeOpts = with lib.types; { config, ... }: {
+  homeOpts = with lib.types; { ... }@args:
+  let
+    config' = args.config;
+  in
+  {
     options = {
       inherit (commonOpts) system nixpkgs home-manager;
       # TODO: docCustom for home-manager?
@@ -53,16 +57,16 @@ let
         description = "home-manager configuration module.";
         type = mkOptionType {
           name = "home-manager configuration";
-          merge = _: defs: mkHome {
-            config' = config;
+          merge = _: defs: applyAssertions config (mkHome {
+            inherit config';
             defs = map (d: inlineModule' d.file d.value) defs;
-          };
+          });
         };
       };
     };
 
     config = {
-      nixpkgs = mkDefault config.home-manager;
+      nixpkgs = mkDefault config'.home-manager;
     };
   };
 in
diff --git a/lib.nix b/lib.nix
index ce6f1a7..c4cdb4d 100644
--- a/lib.nix
+++ b/lib.nix
@@ -1,9 +1,10 @@
 { lib }:
 let
-  inherit (builtins) length match replaceStrings elemAt mapAttrs head split;
+  inherit (builtins) length match replaceStrings elemAt mapAttrs head split filter;
   inherit (lib)
     genAttrs mapAttrs' mapAttrsToList filterAttrsRecursive nameValuePair types
-    mkOption mkOverride mkForce mkIf mergeEqualOption optional hasPrefix;
+    mkOption mkOverride mkForce mkIf mergeEqualOption optional hasPrefix
+    showWarnings concatStringsSep flatten unique;
   inherit (lib.flake) defaultSystems;
 in
 rec {
@@ -69,6 +70,24 @@ rec {
   mkDefault' = mkOverride 900;
   mkVMOverride' = mkOverride 9;
 
+  # This is shocking...
+  duplicates = l:
+    flatten
+      (map
+        (e:
+          optional
+            ((length (filter (e2: e2 == e) l)) > 1)
+            e)
+        (unique l));
+
+  applyAssertions = config: res:
+  let
+    failedAssertions = map (x: x.message) (filter (x: !x.assertion) config.assertions);
+  in
+  if failedAssertions != []
+    then throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}"
+    else showWarnings config.warnings res;
+
   homeStateVersion = hmBranch: {
     # The flake passes a default setting, but we don't care about that
     # Currently don't need any logic here, but we might need to use a newer version later
diff --git a/nixos/default.nix b/nixos/default.nix
index 62ee89d..88b9d9c 100644
--- a/nixos/default.nix
+++ b/nixos/default.nix
@@ -1,8 +1,10 @@
 { lib, pkgsFlakes, hmFlakes, inputs, pkgs', config, ... }:
 let
   inherit (builtins) attrValues mapAttrs;
-  inherit (lib) substring flatten optional optionals mkIf mkDefault mkForce mkOption mkOptionType;
-  inherit (lib.my) naiveIPv4Gateway homeStateVersion mkOpt' mkBoolOpt' mkDefault' commonOpts inlineModule';
+  inherit (lib)
+    substring flatten optional optionals mkIf mkDefault mkForce mkOption mkOptionType;
+  inherit (lib.my)
+    naiveIPv4Gateway homeStateVersion mkOpt' mkBoolOpt' mkDefault' commonOpts inlineModule' applyAssertions duplicates;
 
   cfg = config.nixos;
 
@@ -114,7 +116,11 @@ let
     };
   };
 
-  systemOpts = with lib.types; { name, config, ... }: {
+  systemOpts = with lib.types; { name, ... }@args:
+  let
+    config' = args.config;
+  in
+  {
     options = {
       inherit (commonOpts) system nixpkgs home-manager;
       hmNixpkgs = commonOpts.nixpkgs;
@@ -131,18 +137,17 @@ let
         # Based on the definition of containers.<name>.config
         type = mkOptionType {
           name = "Toplevel NixOS config";
-          merge = _: defs: mkSystem {
-            inherit name;
-            config' = config;
+          merge = _: defs: applyAssertions config (mkSystem {
+            inherit name config';
             defs = map (d: inlineModule' d.file d.value) defs;
-          };
+          });
         };
       };
     };
 
     config = {
-      home-manager = mkDefault config.nixpkgs;
-      hmNixpkgs = mkDefault config.nixpkgs;
+      home-manager = mkDefault config'.nixpkgs;
+      hmNixpkgs = mkDefault config'.nixpkgs;
     };
   };
 in
@@ -156,4 +161,25 @@ in
       systems = mkOpt' (attrsOf (submodule systemOpts)) { } "NixOS systems to be exported by nixfiles.";
     };
   };
+
+  config = {
+    assertions =
+    let
+      assignedIPs =
+        flatten
+          (map
+            (as:
+              map
+                (a: [ a.ipv4.address a.ipv6.address ])
+                (attrValues as))
+            (attrValues allAssignments));
+      dupIPs = duplicates assignedIPs;
+    in
+    [
+      {
+        assertion = dupIPs == [ ];
+        message = "Duplicate assignments: ${toString dupIPs}";
+      }
+    ];
+  };
 }