diff --git a/server/src/__tests__/issue-references-service.test.ts b/server/src/__tests__/issue-references-service.test.ts
index ceec46a6477..3954b5dd5b8 100644
--- a/server/src/__tests__/issue-references-service.test.ts
+++ b/server/src/__tests__/issue-references-service.test.ts
@@ -175,6 +175,49 @@ describeEmbeddedPostgres("issueReferenceService", () => {
     expect(pap3?.sources.map((source) => source.label)).toEqual(["description"]);
   });
 
+  it("serializes concurrent syncs for a description with repeated references", async () => {
+    const companyId = randomUUID();
+    const sourceIssueId = randomUUID();
+    const targetIssueId = randomUUID();
+
+    await db.insert(companies).values({
+      id: companyId,
+      name: "Paperclip Concurrent References",
+      issuePrefix: `C${companyId.replace(/-/g, "").slice(0, 6).toUpperCase()}`,
+      requireBoardApprovalForNewAgents: false,
+    });
+
+    await db.insert(issues).values([
+      {
+        id: sourceIssueId,
+        companyId,
+        title: "Source issue",
+        description: "Coordinate COM-2 and confirm COM-2 again.",
+        status: "todo",
+        priority: "medium",
+        identifier: "COM-1",
+      },
+      {
+        id: targetIssueId,
+        companyId,
+        title: "Target issue",
+        status: "todo",
+        priority: "medium",
+        identifier: "COM-2",
+      },
+    ]);
+
+    await expect(Promise.all(
+      Array.from({ length: 20 }, () => refs.syncIssue(sourceIssueId)),
+    )).resolves.toHaveLength(20);
+
+    const mentions = await db
+      .select()
+      .from(issueReferenceMentions);
+    expect(mentions).toHaveLength(1);
+    expect(mentions[0]?.targetIssueId).toBe(targetIssueId);
+  });
+
   it("backfills existing references for a company without requiring write-time sync", async () => {
     const companyId = randomUUID();
     const sourceIssueId = randomUUID();
diff --git a/server/src/services/issue-references.ts b/server/src/services/issue-references.ts
index 8d08ccba581..5b5a52d419e 100644
--- a/server/src/services/issue-references.ts
+++ b/server/src/services/issue-references.ts
@@ -174,8 +174,8 @@ export function issueReferenceService(db: Db) {
     }
   }
 
-  async function issueById(issueId: string, dbOrTx: any = db) {
-    return dbOrTx
+  async function issueById(issueId: string, dbOrTx: any = db, lock = false) {
+    const query = dbOrTx
       .select({
         id: issues.id,
         companyId: issues.companyId,
@@ -183,13 +183,14 @@ export function issueReferenceService(db: Db) {
         description: issues.description,
       })
       .from(issues)
-      .where(eq(issues.id, issueId))
+      .where(eq(issues.id, issueId));
+    return (lock ? query.for("update") : query)
       .then((rows: Array<{ id: string; companyId: string; title: string; description: string | null }>) => rows[0] ?? null);
   }
 
   async function syncIssue(issueId: string, dbOrTx: any = db) {
     const runSync = async (tx: any) => {
-      const issue = await issueById(issueId, tx);
+      const issue = await issueById(issueId, tx, true);
       if (!issue) throw notFound("Issue not found");
 
       await replaceSourceMentions({