diff --git a/DESIGN.md b/DESIGN.md
index dae767e..2958383 100644
--- a/DESIGN.md
+++ b/DESIGN.md
@@ -137,6 +137,11 @@ development and tests). It can't be left open by omission.
   (unknown → 401) and checks the timestamp is within `maxSkewMs` (default 60 s). It then rejects a
   signature already accepted within that window (replay protection), and verifies the signature
   over the received path and raw body. The body is parsed only afterwards.
+- **Unique timestamps.** Ed25519 signatures are deterministic, so identical requests in the same
+  millisecond would carry the same signature and the second would be rejected as a replay.
+  `RequestAuth.headers` makes timestamps strictly increase per public key within a process. Two
+  processes (e.g. browser tabs) with the same key can still collide, rarely; a nonce in the
+  payload would fix that but changes the signing scheme.
 - **Algorithms are pluggable.** Built in: `Ed25519Signer` / `Ed25519Verifier` (WebCrypto, so no
   dependencies; Node 22+ and current browsers; keys persist as PKCS #8). Others, e.g. secp256k1 via
   libauth, implement `RequestSigner` (`algorithm`, `publicKey`, `sign`) and `SignatureVerifier`
diff --git a/src/utils/request-auth.ts b/src/utils/request-auth.ts
index 87737be..c94f39c 100644
--- a/src/utils/request-auth.ts
+++ b/src/utils/request-auth.ts
@@ -54,9 +54,19 @@ export class RequestAuth {
     return new TextEncoder().encode(`${timestamp}:${path}:${body}`)
   }
 
-  /** Auth headers for a request to `path` with `body`. */
+  /** Last timestamp used per public key in this process (see `headers`). */
+  private static readonly lastTimestamps = new Map<string, number>()
+
+  /**
+   * Auth headers for a request to `path` with `body`. Timestamps per public key strictly increase
+   * within a process: Ed25519 signatures are deterministic, so two identical requests in the same
+   * millisecond would otherwise carry the same signature, and the second would be rejected as a
+   * replay.
+   */
   static async headers(signer: RequestSigner, path: string, body: string, now: number = Date.now()): Promise<Record<string, string>> {
-    const timestamp = String(now)
+    const time = Math.max(now, (RequestAuth.lastTimestamps.get(signer.publicKey) ?? 0) + 1)
+    RequestAuth.lastTimestamps.set(signer.publicKey, time)
+    const timestamp = String(time)
     const signature = await signer.sign(RequestAuth.payload(timestamp, path, body))
     return {
       [AUTH_HEADERS.publicKey]: signer.publicKey,
diff --git a/test/utils/request-auth.test.ts b/test/utils/request-auth.test.ts
index 532f10d..706f07c 100644
--- a/test/utils/request-auth.test.ts
+++ b/test/utils/request-auth.test.ts
@@ -24,7 +24,7 @@ describe('RequestAuth', () => {
     await expect(auth.verify(await signed(signer), '/kv/readMany', '{"keys":["b"]}')).rejects.toThrow(/Invalid signature/)
     await expect(auth.verify(await signed(signer), '/kv/writeMany', '{"keys":["a"]}')).rejects.toThrow(/Invalid signature/)
     const headers = await signed(signer)
-    headers.set(AUTH_HEADERS.timestamp, String(NOW + 1))
+    headers.set(AUTH_HEADERS.timestamp, String(Number(headers.get(AUTH_HEADERS.timestamp)) + 1))
     await expect(auth.verify(headers, '/kv/readMany', '{"keys":["a"]}')).rejects.toThrow(/Invalid signature/)
     const other = await Ed25519Signer.generate()
     const swapped = await signed(signer)
@@ -32,11 +32,20 @@ describe('RequestAuth', () => {
     await expect(auth.verify(swapped, '/kv/readMany', '{"keys":["a"]}')).rejects.toThrow(/Invalid signature/)
   })
 
+  it('accepts identical requests made in the same millisecond (timestamps per key increase)', async () => {
+    const signer = await Ed25519Signer.generate()
+    const sameKey = await Ed25519Signer.fromPrivateKey(await signer.exportPrivateKey())
+    const auth = new RequestAuth([new Ed25519Verifier()], { now: () => NOW })
+    const requests = await Promise.all([signed(signer), signed(signer), signed(sameKey)])
+    expect(new Set(requests.map((h) => h.get(AUTH_HEADERS.signature))).size).toBe(3)
+    for (const headers of requests) expect(await auth.verify(headers, '/kv/readMany', '{"keys":["a"]}')).toBe(signer.publicKey)
+  })
+
   it('rejects old or future timestamps, replays, missing headers and unknown algorithms', async () => {
     const signer = await Ed25519Signer.generate()
     const auth = new RequestAuth([new Ed25519Verifier()], { now: () => NOW, maxSkewMs: 1000 })
-    await expect(auth.verify(await signed(signer, '/p', 'b', NOW - 5000), '/p', 'b')).rejects.toThrow(/window/)
-    await expect(auth.verify(await signed(signer, '/p', 'b', NOW + 5000), '/p', 'b')).rejects.toThrow(/window/)
+    await expect(auth.verify(await signed(await Ed25519Signer.generate(), '/p', 'b', NOW - 5000), '/p', 'b')).rejects.toThrow(/window/)
+    await expect(auth.verify(await signed(await Ed25519Signer.generate(), '/p', 'b', NOW + 5000), '/p', 'b')).rejects.toThrow(/window/)
     const once = await signed(signer, '/p', 'b')
     await auth.verify(once, '/p', 'b')
     await expect(auth.verify(once, '/p', 'b')).rejects.toThrow(/Replayed/)