diff --git a/docs/runbooks/keycloak.md b/docs/runbooks/keycloak.md
index 976f95a..8d71bbf 100644
--- a/docs/runbooks/keycloak.md
+++ b/docs/runbooks/keycloak.md
@@ -63,5 +63,11 @@ Or enrol a phone once: the secret in base32 is `IJEUOTKFIRCVORKSJNCVET2UKBJUKRKE
 (`otpauth://totp/medewerker?secret=IJEUOTKFIRCVORKSJNCVET2UKBJUKRKE`). The e2e computes its
 own code in `tests/e2e/medewerker-login.ts`.
 
+**A code is single-use.** Keycloak's `otpPolicyCodeReusable` defaults to false, so it refuses a
+code it has already accepted — a second login as the same medewerker inside the same 30-second
+window fails with `invalid_grant` / *Invalid user credentials*, even though the code is current.
+Nothing to fix in the realm: wait for the next window, or spend the following counter, which is
+what `nextUnusedCounter` in `tests/e2e/medewerker-login.ts` does for back-to-back specs.
+
 **Fixture only.** A shared, committed secret is a demo convenience, never a production
 posture — see the ADR's consequences.
diff --git a/tests/e2e/medewerker-login.ts b/tests/e2e/medewerker-login.ts
index 93bd879..53ba23a 100644
--- a/tests/e2e/medewerker-login.ts
+++ b/tests/e2e/medewerker-login.ts
@@ -1,4 +1,7 @@
 import { createHmac } from 'node:crypto';
+import { readFileSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
 import type { Page } from '@playwright/test';
 
 // The medewerker realm enforces MFA (S-15c), so a staff login is two steps: password, then a TOTP
@@ -6,22 +9,49 @@ import type { Page } from '@playwright/test';
 // secret bytes — so the e2e can compute a valid code instead of enrolling an authenticator.
 const OTP_SECRET = 'BIGMEDEWERKEROTPSEED';
 
+export const OTP_PERIOD_MS = 30_000;
+
 // RFC 6238 TOTP: HMAC-SHA1 over the 30-second counter, dynamically truncated to 6 digits.
 export function totp(secret = OTP_SECRET, at = Date.now()): string {
   const counter = Buffer.alloc(8);
-  counter.writeBigUInt64BE(BigInt(Math.floor(at / 1000 / 30)));
+  counter.writeBigUInt64BE(BigInt(Math.floor(at / OTP_PERIOD_MS)));
   const mac = createHmac('sha1', secret).update(counter).digest();
   const offset = mac[mac.length - 1] & 0x0f;
   return String((mac.readUInt32BE(offset) & 0x7fffffff) % 1_000_000).padStart(6, '0');
 }
 
+// Keycloak refuses a TOTP code it has already accepted (its otpPolicyCodeReusable defaults to
+// false), so two logins as the same medewerker inside one 30-second window would both submit the
+// same code and the second is rejected. Spend the first counter this medewerker has left.
+export function nextUnusedCounter(now: number, spent: number): number {
+  return Math.max(Math.floor(now / OTP_PERIOD_MS), spent + 1);
+}
+
+// The spent counter lives on disk rather than in module state: Playwright starts a fresh worker
+// process for a retry, which would otherwise forget it and resubmit the rejected code.
+function spendCounter(username: string): number {
+  const file = join(tmpdir(), `otp-counter-${username}`);
+  let spent = -1;
+  try {
+    spent = Number(readFileSync(file, 'utf8')) || -1;
+  } catch {
+    // first login as this medewerker in this run
+  }
+  const counter = nextUnusedCounter(Date.now(), spent);
+  writeFileSync(file, String(counter));
+  return counter;
+}
+
 export async function loginMedewerker(page: Page, username: string): Promise<void> {
   await page.locator('#username').fill(username);
   await page.locator('#password').fill('test123');
   await page.locator('#kc-login').click();
 
-  // Keycloak's conditional-OTP step. Its lookAheadWindow accepts the neighbouring counters, so a
-  // code computed just before a 30-second boundary still validates — no retry needed.
-  await page.locator('#otp').fill(totp());
+  // Keycloak's conditional-OTP step. Wait out the rest of the window if the counter we may spend is
+  // still in the future; its lookAheadWindow would accept the code a moment early, but only by one
+  // counter — waiting keeps a third login in the same window valid too.
+  const counter = spendCounter(username);
+  await page.waitForTimeout(Math.max(0, counter * OTP_PERIOD_MS - Date.now()));
+  await page.locator('#otp').fill(totp(OTP_SECRET, counter * OTP_PERIOD_MS));
   await page.locator('#kc-login').click();
 }