diff --git a/src/Migration/DuplicateEventAliasMigration.php b/src/Migration/DuplicateEventAliasMigration.php
new file mode 100644
index 0000000..acfa474
--- /dev/null
+++ b/src/Migration/DuplicateEventAliasMigration.php
@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Mummert\EventManagerBundle\Migration;
+
+use Contao\CoreBundle\Migration\AbstractMigration;
+use Contao\CoreBundle\Migration\MigrationResult;
+use Doctrine\DBAL\Connection;
+
+/**
+ * Doppelte Event-Aliasse auflösen.
+ *
+ * Das Frontend-Formular hat Aliasse früher ohne Eindeutigkeitsprüfung aus Datum und
+ * Titel gebaut. Bei gleichem Titel und Datum (z. B. nach dem Duplizieren) entstand
+ * derselbe Alias, und nur eines der Events war über seine URL erreichbar.
+ *
+ * Pro Gruppe behält das älteste Event (niedrigste ID) seinen Alias und damit seine
+ * bisherige URL, die übrigen bekommen -2, -3, ... angehängt. Leere Aliasse (Entwürfe)
+ * bleiben unberührt, Contao verwendet dort die ID.
+ */
+class DuplicateEventAliasMigration extends AbstractMigration
+{
+    public function __construct(
+        private readonly Connection $connection,
+    ) {
+    }
+
+    public function getName(): string
+    {
+        return 'Eventmanager: doppelte Event-Aliasse eindeutig machen';
+    }
+
+    public function shouldRun(): bool
+    {
+        if (!$this->connection->createSchemaManager()->tablesExist(['tl_calendar_events'])) {
+            return false;
+        }
+
+        return [] !== $this->findDuplicateAliases();
+    }
+
+    public function run(): MigrationResult
+    {
+        $renamed = [];
+
+        foreach ($this->findDuplicateAliases() as $alias) {
+            $ids = $this->connection->fetchFirstColumn('SELECT id FROM tl_calendar_events WHERE alias = ? ORDER BY id', [$alias]);
+
+            // ältestes Event behält den Alias
+            array_shift($ids);
+            $counter = 2;
+
+            foreach ($ids as $id) {
+                do {
+                    $candidate = $alias.'-'.$counter++;
+                } while (false !== $this->connection->fetchOne('SELECT id FROM tl_calendar_events WHERE alias = ?', [$candidate]));
+
+                $this->connection->update('tl_calendar_events', ['alias' => $candidate, 'tstamp' => time()], ['id' => (int) $id]);
+                $renamed[] = sprintf('ID %d: %s', $id, $candidate);
+            }
+        }
+
+        return $this->createResult(true, 'Umbenannt: '.implode(', ', $renamed));
+    }
+
+    /**
+     * @return list<string>
+     */
+    private function findDuplicateAliases(): array
+    {
+        return $this->connection->fetchFirstColumn(
+            "SELECT alias FROM tl_calendar_events WHERE alias != '' GROUP BY alias HAVING COUNT(*) > 1",
+        );
+    }
+}
diff --git a/src/Service/EventRepository.php b/src/Service/EventRepository.php
index 6e28952..265fcc0 100644
--- a/src/Service/EventRepository.php
+++ b/src/Service/EventRepository.php
@@ -526,7 +526,7 @@ class EventRepository
      */
     public function update(int $eventId, array $data): void
     {
-        $eventData = $this->buildEventData($data);
+        $eventData = $this->buildEventData($data, $eventId);
 
         $this->connection->update(
             'tl_calendar_events',
@@ -631,13 +631,13 @@ class EventRepository
      *
      * @return array<string, mixed>
      */
-    private function buildEventData(array $data): array
+    private function buildEventData(array $data, ?int $eventId = null): array
     {
         $startDate = !empty($data['startDate']) ? strtotime((string) $data['startDate']) : false;
         $endDate = !empty($data['endDate']) ? strtotime((string) $data['endDate']) : false;
         $url = trim((string) ($data['url'] ?? ''));
         $addTime = !empty($data['addTime']);
-        $alias = $this->buildAlias($startDate, (string) ($data['title'] ?? ''));
+        $alias = $this->resolveAlias($startDate, (string) ($data['title'] ?? ''), $eventId);
 
         $startDateTimestamp = false !== $startDate ? (int) $startDate : 0;
         $endDateTimestamp = false !== $endDate ? (int) $endDate : 0;
@@ -678,6 +678,53 @@ class EventRepository
         ];
     }
 
+    /**
+     * Alias für ein Event aus dem Frontend-Formular.
+     *
+     * - Bestehende Events behalten ihren Alias (stabile URLs, auch wenn Titel oder
+     *   Datum korrigiert werden); nur ein leerer Alias wird neu erzeugt, z. B. bei
+     *   einer frisch duplizierten Kopie.
+     * - Der Alias ist immer eindeutig: belegte Aliasse bekommen -2, -3, ... angehängt.
+     */
+    private function resolveAlias(false|int $startDate, string $title, ?int $eventId): string
+    {
+        if (null !== $eventId && $eventId > 0) {
+            $existing = (string) $this->connection->fetchOne('SELECT alias FROM tl_calendar_events WHERE id = ?', [$eventId]);
+
+            if ('' !== $existing && !$this->aliasExists($existing, $eventId)) {
+                return $existing;
+            }
+        }
+
+        $base = $this->buildAlias($startDate, $title);
+
+        if ('' === $base) {
+            return '';
+        }
+
+        return $this->makeUniqueAlias($base, $eventId);
+    }
+
+    private function makeUniqueAlias(string $base, ?int $eventId = null): string
+    {
+        $candidate = $base;
+        $counter = 2;
+
+        while ($this->aliasExists($candidate, $eventId)) {
+            $candidate = $base.'-'.$counter++;
+        }
+
+        return $candidate;
+    }
+
+    private function aliasExists(string $alias, ?int $exceptEventId = null): bool
+    {
+        return false !== $this->connection->fetchOne(
+            'SELECT id FROM tl_calendar_events WHERE alias = ? AND id != ? LIMIT 1',
+            [$alias, (int) $exceptEventId],
+        );
+    }
+
     private function buildAlias(false|int $startDate, string $title): string
     {
         if (false === $startDate || $startDate <= 0) {