hosts/public-access-proxy: add assertion for haproxy backend

hosts/public-access-proxy/proxy.nix

@@ -97,6 +97,15 @@ in
   };
 
   config = lib.mkIf cfg.enable {
+    assertions = [{
+      assertion = let
+        proxyTo = lib.forEach cfg.proxyHosts (x: x.proxyTo.host);
+        uniqueProxyTo = lib.unique proxyTo;
+      in
+        (builtins.length proxyTo) == (builtins.length uniqueProxyTo);
+      message = "proxyTo must be unique across the list of proxyHosts. Insert your domain you want to proxy to the list of hostNames instead.";
+    }];
+
     services.haproxy = {
       enable = true;
       config = ''