diff --git a/src/database/db.ts b/src/database/db.ts
index a6cfa21..f868e16 100644
--- a/src/database/db.ts
+++ b/src/database/db.ts
@@ -47,6 +47,15 @@ export async function getLatestMeasurement(): Promise<Measurement | null> {
   return rows.length > 0 ? (rows[0] as Measurement) : null
 }
 
+export async function getMeasurementByDate(date: string): Promise<Measurement | null> {
+  const day = date.slice(0, 10)
+  const rows = await db!.getAllAsync<any>(
+    "SELECT * FROM measurements WHERE measuredAt LIKE ? LIMIT 1",
+    [`${day}%`]
+  )
+  return rows.length > 0 ? (rows[0] as Measurement) : null
+}
+
 export async function updateMeasurement(id: string, syncStatus: SyncStatus): Promise<void> {
   await db!.runAsync(
     'UPDATE measurements SET syncStatus = ?, updatedAt = ? WHERE id = ?',
diff --git a/src/screens/NewMeasurementScreen.tsx b/src/screens/NewMeasurementScreen.tsx
index b143cab..c1f433a 100644
--- a/src/screens/NewMeasurementScreen.tsx
+++ b/src/screens/NewMeasurementScreen.tsx
@@ -3,7 +3,7 @@ import { View, Text, Alert } from 'react-native'
 import { useFocusEffect } from '@react-navigation/native'
 import MeasurementForm from '../components/MeasurementForm'
 import { Measurement } from '../types'
-import { initDatabase, insertMeasurement, getLatestMeasurement } from '../database/db'
+import { initDatabase, insertMeasurement, getLatestMeasurement, getMeasurementByDate, upsertMeasurement } from '../database/db'
 
 type Props = {
   navigation: any
@@ -29,7 +29,32 @@ export default function NewMeasurementScreen({ navigation }: Props) {
 
   const handleSubmit = async (m: Measurement) => {
     await initDatabase()
-    await insertMeasurement(m)
+    const existing = await getMeasurementByDate(m.measuredAt)
+    if (existing) {
+      Alert.alert(
+        'Dubbele datum',
+        `Er bestaat al een meting van ${new Date(m.measuredAt).toLocaleDateString('nl-NL')}. Overschrijven?`,
+        [
+          { text: 'Annuleren', style: 'cancel' },
+          {
+            text: 'Overschrijven',
+            style: 'destructive',
+            onPress: () => doSave(m, existing.id),
+          },
+        ]
+      )
+      return
+    }
+    doSave(m)
+  }
+
+  const doSave = async (m: Measurement, existingId?: string) => {
+    if (existingId) {
+      m.id = existingId
+      await upsertMeasurement(m)
+    } else {
+      await insertMeasurement(m)
+    }
     try {
       const { syncAll } = await import('../sync/syncService')
       await syncAll()