diff --git a/src/Manifest/ManifestManager/Options/OutTable/Serializer/LegacyManifestNormalizer.php b/src/Manifest/ManifestManager/Options/OutTable/Serializer/LegacyManifestNormalizer.php
index d04cd53..f52b85e 100644
--- a/src/Manifest/ManifestManager/Options/OutTable/Serializer/LegacyManifestNormalizer.php
+++ b/src/Manifest/ManifestManager/Options/OutTable/Serializer/LegacyManifestNormalizer.php
@@ -72,6 +72,7 @@ private function normalizeSchema(ManifestOptions $object, array &$data): void
 
                 $this->normalizeDataTypes($schema, $columnMetadata);
                 $this->normalizeColumnMetadata($schema, $columnMetadata);
+                $this->deduplicateMetadata($columnMetadata);
 
                 if (!empty($columnMetadata)) {
                     $data['column_metadata'][$schema->getName()] = $columnMetadata;
@@ -248,4 +249,23 @@ public function supportsDenormalization($data, $type, $format = null): bool
     {
         return $type === ManifestOptions::class;
     }
+
+    private function deduplicateMetadata(array &$columnMetadata): void
+    {
+        $columnMetadata = array_values(array_reduce(
+            $columnMetadata,
+            /**
+             * @param array<string, array<string, mixed>> $carry
+             * @param array<string, mixed> $item
+             * @return array<string, array<string, mixed>>
+             */
+            function (array $carry, array $item): array {
+                if (isset($item['key']) && !isset($carry[$item['key']])) {
+                    $carry[$item['key']] = $item;
+                }
+                return $carry;
+            },
+            [],
+        ));
+    }
 }
diff --git a/tests/Manifest/ManifestManagerTest.php b/tests/Manifest/ManifestManagerTest.php
index 8e88bb9..37cdd56 100644
--- a/tests/Manifest/ManifestManagerTest.php
+++ b/tests/Manifest/ManifestManagerTest.php
@@ -361,6 +361,10 @@ public function provideWriteManifestOptions(): array
                                 'key' => 'yet.another.key',
                                 'value' => 'Some other value',
                             ],
+                            [
+                                'key' => 'yet.another.key', //duplicated key should be removed
+                                'value' => 'Some other value',
+                            ],
                         ],
                     ])
                     ->setDestination('my.table')