diff --git a/apps/server/src/engine/finance.ts b/apps/server/src/engine/finance.ts
index c614ebcc4..4e7273456 100644
--- a/apps/server/src/engine/finance.ts
+++ b/apps/server/src/engine/finance.ts
@@ -28,6 +28,12 @@ export function analyzeSpending(csv: string) {
   const header = rows.shift()?.map((v) => v.trim().toLowerCase());
   if (!header || !["date", "description", "amount", "category"].every((v) => header.includes(v)))
     throw new Error("CSV needs date,description,amount,category columns");
+  if (
+    ["date", "description", "amount", "category"].some(
+      (name) => header.indexOf(name) !== header.lastIndexOf(name),
+    )
+  )
+    throw new Error("CSV has a duplicate required column");
   if (!rows.length || rows.length > 5000)
     throw new Error("Import between 1 and 5,000 transactions");
   const transactions = rows.map((r, index) => {
diff --git a/tests/finance-headers.test.ts b/tests/finance-headers.test.ts
new file mode 100644
index 000000000..62de874a9
--- /dev/null
+++ b/tests/finance-headers.test.ts
@@ -0,0 +1,40 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { analyzeSpending } from "../apps/server/src/engine/finance.ts";
+
+test("duplicate required CSV headers do not silently discard financial values", () => {
+  for (const column of ["date", "description", "amount", "category"]) {
+    assert.throws(
+      () =>
+        analyzeSpending(
+          `date,description,amount,category,${column}\n2026-09-01,Purchase,10,Food,1000`,
+        ),
+      /duplicate.*column/i,
+    );
+  }
+});
+test("header uniqueness is checked after the same normalization used for lookup", () => {
+  assert.throws(
+    () =>
+      analyzeSpending(
+        "date,description,amount,category, Amount \n2026-09-01,Purchase,10,Food,1000",
+      ),
+    /duplicate.*column/i,
+  );
+});
+test("distinct reordered and extra CSV columns still work", () => {
+  const report = analyzeSpending(
+    "note,category,amount,description,date\nIgnored,Food,10,Purchase,2026-09-01",
+  );
+  assert.equal(report.spending, 10);
+  assert.equal(report.count, 1);
+  assert.equal(report.transactions[0].description, "Purchase");
+});
+
+test("duplicate irrelevant CSV columns remain accepted", () => {
+  const report = analyzeSpending(
+    "note,date,description,amount,category,note\nFirst,2026-09-01,Purchase,10,Food,Second",
+  );
+  assert.equal(report.spending, 10);
+  assert.equal(report.count, 1);
+});