diff --git a/js/data.js b/js/data.js
index 3b8cb63..d517873 100644
--- a/js/data.js
+++ b/js/data.js
@@ -61,11 +61,11 @@ export function toggleClosure(state, isoDate) {
   if (idx >= 0) arr.splice(idx, 1); else arr.push(isoDate);
 }
 
-// Adds entries to history, replacing any existing entries for the same dates
+// Adds entries to history, replacing any existing entries with the same (date, employeeId)
 export function addHistoryEntries(state, entries) {
   if (!entries?.length) return;
-  const dates = new Set(entries.map(e => e.date));
-  state.history = state.history.filter(h => !dates.has(h.date));
+  const keys = new Set(entries.map(e => e.date + '|' + e.employeeId));
+  state.history = state.history.filter(h => !keys.has(h.date + '|' + h.employeeId));
   state.history.push(...entries);
   state.history.sort((a, b) => a.date.localeCompare(b.date));
 }
diff --git a/package.json b/package.json
index 7c5c2f8..db770df 100644
--- a/package.json
+++ b/package.json
@@ -5,6 +5,6 @@
   "type": "module",
   "scripts": {
     "start": "npx serve . -p 3001",
-    "test": "node tests/algorithm.test.mjs"
+    "test": "node tests/algorithm.test.mjs && node tests/data.test.mjs"
   }
 }
diff --git a/tests/data.test.mjs b/tests/data.test.mjs
new file mode 100644
index 0000000..c9c473f
--- /dev/null
+++ b/tests/data.test.mjs
@@ -0,0 +1,47 @@
+import assert from 'node:assert/strict';
+import { addHistoryEntries, removeHistoryEntry } from '../js/data.js';
+
+function makeState(history = []) {
+  return {
+    employees: [],
+    calendar: { holidays: [], companyClosures: [] },
+    history: [...history]
+  };
+}
+
+// addHistoryEntries: two entries with same date but different employees both kept
+{
+  const state = makeState();
+  addHistoryEntries(state, [
+    { date: '2026-04-01', employeeId: 'alice', manual: false },
+    { date: '2026-04-01', employeeId: 'bob', manual: false }
+  ]);
+  assert.equal(state.history.length, 2, 'two distinct entries kept on same date');
+  const ids = state.history.map(h => h.employeeId).sort();
+  assert.deepEqual(ids, ['alice', 'bob'], 'both employees present');
+}
+
+// addHistoryEntries: same (date, employeeId) replaced, others on same date untouched
+{
+  const state = makeState([
+    { date: '2026-04-01', employeeId: 'alice', manual: false },
+    { date: '2026-04-01', employeeId: 'bob', manual: false }
+  ]);
+  addHistoryEntries(state, [
+    { date: '2026-04-01', employeeId: 'alice', manual: true }
+  ]);
+  assert.equal(state.history.length, 2, 'still two entries');
+  const alice = state.history.find(h => h.employeeId === 'alice');
+  assert.equal(alice.manual, true, 'alice replaced with manual=true');
+  const bob = state.history.find(h => h.employeeId === 'bob');
+  assert.equal(bob.manual, false, 'bob untouched');
+}
+
+// addHistoryEntries: empty input → no change
+{
+  const state = makeState([{ date: '2026-04-01', employeeId: 'alice', manual: false }]);
+  addHistoryEntries(state, []);
+  assert.equal(state.history.length, 1, 'empty entries → no change');
+}
+
+console.log('All data tests passed.');