diff --git a/src/Mapping.php b/src/Mapping.php
index 9227b8e..9c3b123 100644
--- a/src/Mapping.php
+++ b/src/Mapping.php
@@ -538,30 +538,30 @@ private function map(array $data)
         foreach ($this->definition->getProperties() as $property) {
             $mapping = new static($this->db, $property->getDefinition(), [$property->getForeignColumn()]);
 
+            $localValues = array_unique(array_filter(array_column($data, $property->getLocalColumn()), fn($v) => $v !== null));
+
             if ($property->getJoinTable()) {
                 $mapping->columns[] = sprintf('%s.%s', $property->getJoinTable(), $property->getJoinForeignColumn());
                 $mapping
                     ->join($property->getJoinTable(), $property->getJoinLocalColumn(), $property->getForeignColumn())
-                    ->in(sprintf('%s.%s', $property->getJoinTable(), $property->getJoinForeignColumn()), array_column($data, $property->getLocalColumn()));
+                    ->in(sprintf('%s.%s', $property->getJoinTable(), $property->getJoinForeignColumn()), $localValues);
                 $groupColumn = $property->getJoinForeignColumn();
             } else {
-                $mapping->in($property->getForeignColumn(), array_column($data, $property->getLocalColumn()));
+                $mapping->in($property->getForeignColumn(), $localValues);
                 $groupColumn = $property->getForeignColumn();
             }
 
             $results = $mapping
                 ->findAll();
 
-            $properties[$property->getName()] = Collection::group($results, function ($result) use ($groupColumn) {
-                return $result[$groupColumn];
-            });
+            $properties[$property->getName()] = array_map(fn($group) => array_values($group), Collection::group($results, fn ($result) => $result[$groupColumn]));
         }
 
         $mapped = [];
         foreach ($data as $item) {
             foreach ($this->definition->getProperties() as $property) {
-                $value = $properties[$property->getName()][$item[$property->getLocalColumn()]] ?? [];
-                $value = array_values($value);
+                $localValue = $item[$property->getLocalColumn()];
+                $value = $localValue !== null ? ($properties[$property->getName()][$localValue] ?? []) : [];
 
                 if (!$property->isCollection()) {
                     $value = array_shift($value);
diff --git a/tests/MappingTest.php b/tests/MappingTest.php
index b7d31dc..56d0a21 100644
--- a/tests/MappingTest.php
+++ b/tests/MappingTest.php
@@ -560,6 +560,75 @@ public function testFindOneWithDirectAndSecondaryLeft()
         $this->assertEquals(400, $customer['orders'][0]['items'][1]['amount']);
     }
 
+    public function testFindAllWithDuplicateLocalColumnValues()
+    {
+        $this->db->execute('CREATE TABLE categories (id INTEGER PRIMARY KEY, label TEXT)');
+        $this->db->execute('CREATE TABLE products (id INTEGER PRIMARY KEY, category_id INTEGER, name TEXT)');
+        $this->db->execute("INSERT INTO categories (id, label) VALUES (1, 'Electronics')");
+        $this->db->execute("INSERT INTO products (id, category_id, name) VALUES (1, 1, 'Phone'), (2, 1, 'Laptop'), (3, NULL, 'Unknown')");
+
+        $category = (new Definition('categories'))->withColumns('id', 'label');
+        $product = (new Definition('products'))
+            ->withColumns('id', 'category_id', 'name')
+            ->withOne($category, 'category', 'id', 'category_id');
+
+        $mapping = new Mapping($this->db, $product);
+        $products = $mapping->findAll();
+
+        $this->assertCount(3, $products);
+        $this->assertEquals('Electronics', $products[0]['category']['label']);
+        $this->assertEquals('Electronics', $products[1]['category']['label']);
+        $this->assertNull($products[2]['category']);
+    }
+
+    public function testFindAllWithManyAndNullableLocalColumn()
+    {
+        $this->db->execute('CREATE TABLE sections (id INTEGER PRIMARY KEY, name TEXT)');
+        $this->db->execute('CREATE TABLE articles (id INTEGER PRIMARY KEY, section_id INTEGER, title TEXT)');
+        $this->db->execute('CREATE TABLE tags (id INTEGER PRIMARY KEY, section_id INTEGER, label TEXT)');
+        $this->db->execute("INSERT INTO sections (id, name) VALUES (1, 'Tech')");
+        $this->db->execute("INSERT INTO articles (id, section_id, title) VALUES (1, 1, 'PHP 8.5'), (2, NULL, 'Unclassified')");
+        $this->db->execute("INSERT INTO tags (id, section_id, label) VALUES (1, 1, 'php'), (2, 1, 'programming')");
+
+        $tag = (new Definition('tags'))->withColumns('id', 'section_id', 'label');
+        $article = (new Definition('articles'))
+            ->withColumns('id', 'section_id', 'title')
+            ->withMany($tag, 'tags', 'section_id', 'section_id');
+
+        $mapping = new Mapping($this->db, $article);
+        $articles = $mapping->findAll();
+
+        $this->assertCount(2, $articles);
+        $this->assertCount(2, $articles[0]['tags']);
+        $this->assertEquals('php', $articles[0]['tags'][0]['label']);
+        $this->assertEquals('programming', $articles[0]['tags'][1]['label']);
+        $this->assertCount(0, $articles[1]['tags']);
+    }
+
+    public function testFindAllWithNullableLocalColumn()
+    {
+        $this->db->execute('CREATE TABLE categories (id INTEGER PRIMARY KEY, label TEXT)');
+        $this->db->execute('CREATE TABLE products (id INTEGER PRIMARY KEY, category_id INTEGER, name TEXT)');
+        $this->db->execute("INSERT INTO categories (id, label) VALUES (1, 'Electronics')");
+        $this->db->execute("INSERT INTO products (id, category_id, name) VALUES (1, 1, 'Phone'), (2, NULL, 'Unknown')");
+
+        $category = (new Definition('categories'))->withColumns('id', 'label');
+        $product = (new Definition('products'))
+            ->withColumns('id', 'category_id', 'name')
+            ->withOne($category, 'category', 'id', 'category_id');
+
+        $mapping = new Mapping($this->db, $product);
+        $products = $mapping->findAll();
+
+        $this->assertCount(2, $products);
+
+        $this->assertEquals('Phone', $products[0]['name']);
+        $this->assertEquals('Electronics', $products[0]['category']['label']);
+
+        $this->assertEquals('Unknown', $products[1]['name']);
+        $this->assertNull($products[1]['category']);
+    }
+
     /**
      * Returns a new mapping for testing.
      *