diff --git a/src/routes/api/the-nook/lifecycle.test.ts b/src/routes/api/the-nook/lifecycle.test.ts
index 25b27e7..31a38f0 100644
--- a/src/routes/api/the-nook/lifecycle.test.ts
+++ b/src/routes/api/the-nook/lifecycle.test.ts
@@ -19,11 +19,15 @@ let trialRows: Record<string, unknown>[] = [];
 let licenseRows: Record<string, unknown>[] = [];
 const updates: { sql: string; args?: unknown[] }[] = [];
 const sent: string[] = [];
+// When true, conditional-claim UPDATEs report rowsAffected 0 (another
+// invocation won the claim first).
+let racingClaim = false;
 
 const fakeConn = {
   execute: async (q: { sql: string; args?: unknown[] } | string) => {
-    const { sql, args } =
+    const { sql: rawSql, args } =
       typeof q === "string" ? { sql: q, args: undefined } : q;
+    const sql = rawSql.trim();
     if (sql.startsWith("SELECT fingerprint")) {
       return { rows: trialRows, rowsAffected: 0 };
     }
@@ -37,7 +41,8 @@ const fakeConn = {
     }
     if (sql.startsWith("UPDATE")) {
       updates.push({ sql, args });
-      return { rows: [], rowsAffected: 1 };
+      const claimed = racingClaim ? 0 : 1;
+      return { rows: [], rowsAffected: claimed };
     }
     return { rows: [], rowsAffected: 0 };
   }
@@ -154,6 +159,21 @@ describe("GET /api/the-nook/lifecycle", () => {
     ).toBe(true);
   });
 
+  it("claims via conditional update — a lost claim never sends", async () => {
+    // Simulate a racing invocation that already claimed the row: the
+    // conditional UPDATE reports rowsAffected 0, and no email goes out.
+    trialRows = [
+      { fingerprint: UUID, email: "raced@example.com", started_at: daysAgo(12) }
+    ];
+    licenseRows = [];
+    racingClaim = true;
+    const res = await GET(request("Bearer sekrit"));
+    const body = await res.json();
+    expect(body.reminders).toBe(0);
+    expect(sent).toHaveLength(0);
+    racingClaim = false;
+  });
+
   it("asks for a testimonial at D+3 but not a review until D+14", async () => {
     licenseRows = [
       {
diff --git a/src/routes/api/the-nook/lifecycle.ts b/src/routes/api/the-nook/lifecycle.ts
index 5f72c48..8b79475 100644
--- a/src/routes/api/the-nook/lifecycle.ts
+++ b/src/routes/api/the-nook/lifecycle.ts
@@ -53,10 +53,18 @@ async function runLifecycle(): Promise<{
     started_at: string;
   }[]) {
     if (ageDays(row.started_at) < 11) continue;
-    await conn.execute({
-      sql: "UPDATE trials SET reminder_sent_at = ? WHERE fingerprint = ?",
+    // Claim-then-send: the conditional UPDATE wins exactly once even when
+    // two invocations race (cron + manual hit) — the loser sees
+    // rowsAffected 0 and never sends. An unconditional mark-then-send let
+    // both invocations pass the SELECT and mail the same user twice.
+    const claim = await conn.execute({
+      sql: `
+        UPDATE trials SET reminder_sent_at = ?
+        WHERE fingerprint = ? AND reminder_sent_at IS NULL
+      `,
       args: [new Date().toISOString(), row.fingerprint]
     });
+    if (claim.rowsAffected === 0) continue;
     // A trial that ended in a purchase gets no buy reminder. The address
     // comparison is on the raw stored values; both writes lowercase.
     const owned = await conn.execute({
@@ -83,18 +91,26 @@ async function runLifecycle(): Promise<{
     if (age < 0) continue;
 
     if (age >= 3 && !row.testimonial_asked_at) {
-      await conn.execute({
-        sql: "UPDATE licenses SET testimonial_asked_at = ? WHERE id = ?",
+      const claim = await conn.execute({
+        sql: `
+          UPDATE licenses SET testimonial_asked_at = ?
+          WHERE id = ? AND testimonial_asked_at IS NULL
+        `,
         args: [new Date().toISOString(), row.id]
       });
+      if (claim.rowsAffected === 0) continue;
       if (await emailTestimonialAsk(row.email)) counts.testimonials++;
     }
 
     if (age >= 14 && !row.review_asked_at) {
-      await conn.execute({
-        sql: "UPDATE licenses SET review_asked_at = ? WHERE id = ?",
+      const claim = await conn.execute({
+        sql: `
+          UPDATE licenses SET review_asked_at = ?
+          WHERE id = ? AND review_asked_at IS NULL
+        `,
         args: [new Date().toISOString(), row.id]
       });
+      if (claim.rowsAffected === 0) continue;
       if (await emailReviewAsk(row.email)) counts.reviews++;
     }
   }