diff --git a/lib/emails/__tests__/processAndSendEmail.test.ts b/lib/emails/__tests__/processAndSendEmail.test.ts
index 774998a3..58e781e2 100644
--- a/lib/emails/__tests__/processAndSendEmail.test.ts
+++ b/lib/emails/__tests__/processAndSendEmail.test.ts
@@ -39,6 +39,7 @@ describe("processAndSendEmail", () => {
         subject: "Test",
         html: expect.stringContaining("Hello world"),
       }),
+      undefined,
     );
   });
 
@@ -56,6 +57,7 @@ describe("processAndSendEmail", () => {
       expect.objectContaining({
         html: expect.stringContaining("<h1>HTML body</h1>"),
       }),
+      undefined,
     );
   });
 
@@ -72,6 +74,7 @@ describe("processAndSendEmail", () => {
       expect.objectContaining({
         cc: ["cc@example.com"],
       }),
+      undefined,
     );
   });
 
@@ -91,6 +94,7 @@ describe("processAndSendEmail", () => {
       expect.objectContaining({
         html: expect.stringContaining("Test Artist"),
       }),
+      undefined,
     );
   });
 
@@ -129,4 +133,24 @@ describe("processAndSendEmail", () => {
       expect(result.error).toContain("Rate limited");
     }
   });
+
+  it("forwards idempotencyKey to Resend so a replayed send delivers once (chat#1918)", async () => {
+    mockSendEmailWithResend.mockResolvedValue({ id: "resend-1" });
+    await processAndSendEmail({
+      to: ["a@b.com"],
+      subject: "S",
+      text: "body",
+      idempotencyKey: "chat_1:msg_2",
+    });
+    expect(mockSendEmailWithResend).toHaveBeenCalledWith(
+      expect.anything(),
+      expect.objectContaining({ idempotencyKey: "chat_1:msg_2" }),
+    );
+  });
+
+  it("omits Resend options entirely when no idempotencyKey is given", async () => {
+    mockSendEmailWithResend.mockResolvedValue({ id: "resend-2" });
+    await processAndSendEmail({ to: ["a@b.com"], subject: "S", text: "body" });
+    expect(mockSendEmailWithResend).toHaveBeenCalledWith(expect.anything(), undefined);
+  });
 });
diff --git a/lib/emails/processAndSendEmail.ts b/lib/emails/processAndSendEmail.ts
index f3e4a445..5c218317 100644
--- a/lib/emails/processAndSendEmail.ts
+++ b/lib/emails/processAndSendEmail.ts
@@ -14,6 +14,12 @@ export interface ProcessAndSendEmailInput {
   html?: string;
   headers?: Record<string, string>;
   room_id?: string;
+  /**
+   * Makes the send exactly-once for a retried or replayed caller. Scheduled
+   * runs re-execute on crash or redeploy and would otherwise deliver the same
+   * report again (chat#1918). Forwarded to Resend, which dedupes on it.
+   */
+  idempotencyKey?: string;
 }
 
 export interface ProcessAndSendEmailSuccess {
@@ -39,7 +45,7 @@ export type ProcessAndSendEmailResult = ProcessAndSendEmailSuccess | ProcessAndS
 export async function processAndSendEmail(
   input: ProcessAndSendEmailInput,
 ): Promise<ProcessAndSendEmailResult> {
-  const { to, cc = [], subject, text, html = "", headers = {}, room_id } = input;
+  const { to, cc = [], subject, text, html = "", headers = {}, room_id, idempotencyKey } = input;
 
   const roomData = room_id ? await selectRoomWithArtist(room_id) : null;
   const footer = getEmailFooter(room_id, roomData?.artist_name || undefined);
@@ -51,14 +57,17 @@ export async function processAndSendEmail(
   // card, DESIGN.md font stack, and the existing footer as the layout footer.
   const htmlWithLayout = renderEmailLayout({ bodyHtml, footerHtml: footer });
 
-  const result = await sendEmailWithResend({
-    from: RECOUP_FROM_EMAIL,
-    to,
-    cc: cc.length > 0 ? cc : undefined,
-    subject,
-    html: htmlWithLayout,
-    headers,
-  });
+  const result = await sendEmailWithResend(
+    {
+      from: RECOUP_FROM_EMAIL,
+      to,
+      cc: cc.length > 0 ? cc : undefined,
+      subject,
+      html: htmlWithLayout,
+      headers,
+    },
+    idempotencyKey ? { idempotencyKey } : undefined,
+  );
 
   if (result instanceof NextResponse) {
     const data = await result.json();
diff --git a/lib/emails/sendEmailHandler.ts b/lib/emails/sendEmailHandler.ts
index 6fcc3ab5..767ece8b 100644
--- a/lib/emails/sendEmailHandler.ts
+++ b/lib/emails/sendEmailHandler.ts
@@ -30,6 +30,7 @@ export async function sendEmailHandler(request: NextRequest): Promise<NextRespon
       headers = {},
       chat_id,
       accountId,
+      idempotency_key,
     } = validated.data;
     const result = await processAndSendEmail({
       to,
@@ -39,6 +40,7 @@ export async function sendEmailHandler(request: NextRequest): Promise<NextRespon
       html,
       headers,
       room_id: chat_id,
+      idempotencyKey: idempotency_key,
     });
 
     if (result.success === false) {
diff --git a/lib/emails/validateSendEmailBody.ts b/lib/emails/validateSendEmailBody.ts
index 3b88a94c..2449d6a4 100644
--- a/lib/emails/validateSendEmailBody.ts
+++ b/lib/emails/validateSendEmailBody.ts
@@ -19,6 +19,10 @@ export const sendEmailBodySchema = z
     html: z.string().optional(),
     headers: z.record(z.string(), z.string()).default({}).optional(),
     chat_id: z.string().optional(),
+    idempotency_key: z
+      .string()
+      .max(256, "idempotency_key must be at most 256 characters")
+      .optional(),
     account_id: z.string().uuid("account_id must be a valid UUID").optional(),
   })
   // Guard: never send an empty/footer-only email. A malformed or empty body