diff --git a/orchestrator/templates/page-passkey.js b/orchestrator/templates/page-passkey.js
index d5fb085..5f2fefd 100644
--- a/orchestrator/templates/page-passkey.js
+++ b/orchestrator/templates/page-passkey.js
@@ -32,7 +32,9 @@
     }
   }
 
-  async function signedProof(path, fields, label) {
+  let signing = false;
+
+  async function prove(path, fields, label) {
     const result = await sendJson(path, fields);
     if (!result.ok || !result.data) {
       toast(`${label}: not possible`, fail(result), false);
@@ -45,6 +47,19 @@
     return signed ? {proof: {payload: result.data.payload, assertion: signed}} : null;
   }
 
+  async function signedProof(path, fields, label) {
+    if (signing) {
+      toast(`${label}: not signed`, ["Another passkey signature is still open: finish or cancel it first; nothing was sent."], false);
+      return null;
+    }
+    signing = true;
+    try {
+      return await prove(path, fields, label);
+    } finally {
+      signing = false;
+    }
+  }
+
   function proofFor(entries, label) {
     return signedProof("/api/sign", {items: entries}, label);
   }
diff --git a/tests/test_hub_passkey_page.py b/tests/test_hub_passkey_page.py
new file mode 100644
index 0000000..6e1f8b1
--- /dev/null
+++ b/tests/test_hub_passkey_page.py
@@ -0,0 +1,67 @@
+from __future__ import annotations
+
+import json
+import subprocess
+import unittest
+
+from hub_support import HUB
+
+PASSKEY = HUB / "templates" / "page-passkey.js"
+PROBE = """
+const source = require("fs").readFileSync(process.argv[1], "utf8");
+const toasts = [];
+const gates = [];
+let asked = 0;
+let served = 0;
+const window = {location: {hostname: "localhost", port: "8737", pathname: "/", hash: ""}};
+const navigator = {credentials: {get: () => {
+  asked += 1;
+  return new Promise((resolve, reject) => gates.push({resolve, reject}));
+}}};
+const sendJson = async () => {
+  served += 1;
+  return {ok: true, data: {payload: {nonce: served}, challenge: "AAAA", credentials: ["AAAA"]}};
+};
+const toast = (title) => toasts.push(title);
+const bytes = new Uint8Array([1]).buffer;
+const credential = {rawId: bytes, response: {clientDataJSON: bytes, authenticatorData: bytes, signature: bytes}};
+const P = new Function("window", "navigator", "sendJson", "toast", "fail", "h", `${source}\\nreturn {signedProof};`)(window, navigator, sendJson, toast, () => [], () => null);
+const tick = () => new Promise((resolve) => setImmediate(resolve));
+const nonce = (signed) => (signed ? signed.proof.payload.nonce : null);
+(async () => {
+  const first = P.signedProof("/api/board/sign", {action: "resume"}, "Resume");
+  const second = P.signedProof("/api/board/sign", {action: "resume"}, "Resume");
+  await tick();
+  const open = {asked, served, toasts: [...toasts]};
+  gates.splice(0).forEach((gate) => gate.resolve(credential));
+  const both = [nonce(await first), nonce(await second)];
+  const canceled = P.signedProof("/api/board/sign", {action: "drain"}, "Drain");
+  await tick();
+  gates.splice(0).forEach((gate) => gate.reject(new Error("canceled")));
+  const dropped = nonce(await canceled);
+  const after = P.signedProof("/api/board/sign", {action: "suspend"}, "Suspend");
+  await tick();
+  gates.splice(0).forEach((gate) => gate.resolve(credential));
+  console.log(JSON.stringify({open, both, canceled: dropped, after: nonce(await after), asked, toasts}));
+})();
+"""
+
+
+class TestPasskeyPage(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        done = subprocess.run(["node", "-e", PROBE, str(PASSKEY)], capture_output=True, text=True, timeout=60, check=True)
+        cls.data = json.loads(done.stdout)
+
+    def test_a_second_signature_while_one_is_open_asks_no_passkey_and_sends_nothing(self):
+        self.assertEqual(self.data["open"], {"asked": 1, "served": 1, "toasts": ["Resume: not signed"]})
+        self.assertEqual(self.data["both"], [1, None])
+
+    def test_a_canceled_signature_frees_the_next_one(self):
+        self.assertIsNone(self.data["canceled"])
+        self.assertEqual((self.data["after"], self.data["asked"]), (3, 3))
+        self.assertEqual(self.data["toasts"], ["Resume: not signed", "Drain: not signed"])
+
+
+if __name__ == "__main__":
+    unittest.main()