diff --git a/database/migrations/2026_04_12_000000_add_unique_constraint_to_train_checkins.php b/database/migrations/2026_04_12_000000_add_unique_constraint_to_train_checkins.php
new file mode 100644
index 000000000..c0272ff97
--- /dev/null
+++ b/database/migrations/2026_04_12_000000_add_unique_constraint_to_train_checkins.php
@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * Re-adds a unique constraint on (user_id, trip_id, origin_stopover_id) to prevent
+ * duplicate checkins. The old constraint (user_trip_origin_departure) was dropped in
+ * 2024_05_22_000003 when the legacy `origin` column was removed.
+ *
+ * Before adding the constraint, existing duplicate rows are cleaned up: for each
+ * duplicate group the oldest checkin (lowest id) is kept; newer duplicates and their
+ * corresponding statuses are deleted.
+ */
+return new class() extends Migration
+{
+    public function up(): void
+    {
+        $duplicates = DB::select(
+            'SELECT MIN(id) AS keep_id, user_id, trip_id, origin_stopover_id
+             FROM train_checkins
+             WHERE origin_stopover_id IS NOT NULL
+             GROUP BY user_id, trip_id, origin_stopover_id
+             HAVING COUNT(*) > 1'
+        );
+
+        foreach ($duplicates as $duplicate) {
+            $toDelete = DB::table('train_checkins')
+                ->where('user_id', $duplicate->user_id)
+                ->where('trip_id', $duplicate->trip_id)
+                ->where('origin_stopover_id', $duplicate->origin_stopover_id)
+                ->where('id', '!=', $duplicate->keep_id)
+                ->pluck('status_id');
+
+            DB::table('statuses')
+                ->whereIn('id', $toDelete)
+                ->delete();
+        }
+
+        Schema::table('train_checkins', static function (Blueprint $table) {
+            $table->unique(
+                ['user_id', 'trip_id', 'origin_stopover_id'],
+                'user_trip_origin_stopover_unique'
+            );
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('train_checkins', static function (Blueprint $table) {
+            $table->dropUnique('user_trip_origin_stopover_unique');
+        });
+    }
+};