Prevent flushing same tags several times

lib/internal/Magento/Framework/Indexer/CacheContext.php

@@ -73,6 +73,6 @@ public function getIdentities()
                 $identities[] = $cacheTag . '_' . $id;
             }
         }
-        return array_merge($identities, array_unique($this->tags));
+        return array_unique(array_merge($identities, array_unique($this->tags)));
     }
 }

lib/internal/Magento/Framework/Indexer/Test/Unit/CacheContextTest.php

@@ -0,0 +1,58 @@
+<?php
+
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\Framework\Indexer\Test\Unit;
+
+use Magento\Framework\Indexer\CacheContext;
+use PHPUnit\Framework\TestCase;
+
+class CacheContextTest extends TestCase
+{
+    /**
+     * @var Batch
+     */
+    private $object;
+
+    protected function setUp(): void
+    {
+        $this->object = new CacheContext();
+    }
+
+    /**
+     * @param array $tagsData
+     * @param array $expected
+     * @dataProvider getTagsDataProvider
+     */
+    public function testUniqueTags($tagsData, $expected)
+    {
+        foreach ($tagsData as $tagSet) {
+            foreach ($tagSet as $cacheTag => $ids) {
+                $this->object->registerEntities($cacheTag, $ids);
+            }
+        }
+
+        $this->assertEquals($this->object->getIdentities(), $expected);
+    }
+
+    /**
+     * @return array
+     */
+    public function getTagsDataProvider()
+    {
+        return [
+            'same entities and ids' => [
+                [['cat_p' => [1]], ['cat_p' => [1]]],
+                ['cat_p_1']
+            ],
+            'same entities with overlapping ids' => [
+                [['cat_p' => [1, 2, 3]], ['cat_p' => [3]]],
+                ['cat_p_1', 'cat_p_2', 'cat_p_3']
+            ]
+        ];
+    }
+}