diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 609e921..6f78b6e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -41,3 +41,7 @@ jobs:
       - name: "Functional tests with postgres (nightly or pull_request)"
         if: ${{ always() && (github.event_name == 'schedule' || github.event_name == 'pull_request' ) }}
         run: Build/Scripts/runTests.sh -p ${{ matrix.php }} -d postgres -s functional
+
+      - name: "Functional tests with mysql (nightly or pull_request)"
+        if: ${{ always() && (github.event_name == 'schedule' || github.event_name == 'pull_request' ) }}
+        run: Build/Scripts/runTests.sh -p ${{ matrix.php }} -d mysql -s functional
diff --git a/Classes/Command/UnduplicateCommand.php b/Classes/Command/UnduplicateCommand.php
index 92d7859..c8e3336 100644
--- a/Classes/Command/UnduplicateCommand.php
+++ b/Classes/Command/UnduplicateCommand.php
@@ -195,6 +195,11 @@ protected function execute(InputInterface $input, OutputInterface $output): int
     }
 
     /**
+     * Uses GROUP BY BINARY identifier,storage to make sure we don't get results for identifiers which are only duplicate
+     * if checked case-insensitively.
+     *
+     * Database may be case-insensitive, e.g. charset 'utf8mb5', collation 'utf8mb4_unicode_ci'.
+     *
      * @param mixed $onlyThisIdentifier
      * @param int $onlyThisStorage
      * @return Result
@@ -203,9 +208,7 @@ public function findDuplicates(mixed $onlyThisIdentifier, int $onlyThisStorage):
     {
         $queryBuilder = $this->connectionPool->getQueryBuilderForTable('sys_file');
         $queryBuilder->count('*')
-            ->addSelect('identifier', 'storage')
             ->from('sys_file')
-            ->groupBy('identifier', 'storage')
             ->having('COUNT(*) > 1');
         $whereExpressions = [];
         if ($onlyThisIdentifier) {
@@ -223,27 +226,47 @@ public function findDuplicates(mixed $onlyThisIdentifier, int $onlyThisStorage):
         if ($whereExpressions) {
             $queryBuilder->where(...$whereExpressions);
         }
+
+        $concreteQueryBuilder = $queryBuilder->getConcreteQueryBuilder();
+
+        // GROUP BY BINARY `identifier`,`storage
+        $concreteQueryBuilder->groupBy('BINARY ' . $queryBuilder->quoteIdentifier('identifier'));
+        $concreteQueryBuilder->addGroupBy($queryBuilder->quoteIdentifier('storage'));
+        // SELECT MAX(`identifier`) AS identifier,`storage`
+        $concreteQueryBuilder->addSelect('MAX(' . $queryBuilder->quoteIdentifier('identifier')
+            . ') AS identifier, ' . $queryBuilder->quoteIdentifier('storage'));
+
+        $this->output->writeln('sql=' . $queryBuilder->getSQL(), OutputInterface::VERBOSITY_VERBOSE);
+
         $statement = $queryBuilder
             ->executeQuery();
         return $statement;
     }
 
+    /**
+     * Must make sure we compare identifier case-sensitively, so using "BINARY identifier" here .
+     *
+     * Database may be case-insensitive, e.g. charset 'utf8mb5', collation 'utf8mb4_unicode_ci'.
+     */
     private function findDuplicateFilesForIdentifier(string $identifier, int $storage): array
     {
         $fileQueryBuilder = $this->connectionPool->getQueryBuilderForTable('sys_file');
 
-        return $fileQueryBuilder->select('uid', 'identifier')
+        $fileQueryBuilder->select('uid', 'identifier')
             ->from('sys_file')
             ->where(
-                $fileQueryBuilder->expr()->eq(
-                    'identifier',
-                    $fileQueryBuilder->createNamedParameter($identifier, \PDO::PARAM_STR)
-                ),
                 $fileQueryBuilder->expr()->eq(
                     'storage',
                     $fileQueryBuilder->createNamedParameter($storage, Connection::PARAM_INT)
                 )
-            )->orderBy('uid', 'DESC')->executeQuery()
+            )->orderBy('uid', 'DESC');
+
+        $whereClause = $fileQueryBuilder->expr()->eq('identifier',
+            $fileQueryBuilder->createNamedParameter($identifier, \PDO::PARAM_STR));
+        $whereClause = 'BINARY ' . $whereClause;
+        $fileQueryBuilder->add('where', $whereClause);
+
+        return $fileQueryBuilder->executeQuery()
             ->fetchAllAssociative();
     }
 
diff --git a/README.md b/README.md
index 913f481..e624a72 100644
--- a/README.md
+++ b/README.md
@@ -4,9 +4,23 @@ Finds and fixes duplicates of sys_file entries pointing to the same file. Merges
 Tested successfully with TYPO3 v12.
 
 ## Warning
+
 Older versions for TYPO3 v8 may not consider identifiers with mixed case or sys_file
 entries on several storages (sys_file.storage) correctly, see issue https://github.com/ElementareTeilchen/unduplicator/issues/2
 
+## Portabilty (database)
+
+In order to test for duplicates, a database command like this is used:
+
+```sql
+SELECT COUNT(*), MAX(identifier) AS identifier, storage FROM `sys_file` GROUP BY BINARY identifier, storage HAVING COUNT(*) > 1;
+```
+
+Therefore, it is necessary, that the underlying database engines support MAX and BINARY. This command was tested with the following:
+
+* MariaDB
+* MySQL
+
 ## Usage
 We strongly recommend to run the **reference index update** (before and after):
 If not run before or the references are out of date, some references may be overlooked and a sys_file entry deleted which has references.
diff --git a/Tests/Functional/Command/DataSet/sys_file_duplicates_mix_casesensitive.csv b/Tests/Functional/Command/DataSet/sys_file_duplicates_mix_casesensitive.csv
new file mode 100644
index 0000000..4e71b38
--- /dev/null
+++ b/Tests/Functional/Command/DataSet/sys_file_duplicates_mix_casesensitive.csv
@@ -0,0 +1,5 @@
+"sys_file",,,
+,"uid","identifier","storage"
+,1,"/test/ABC.jpg",1
+,2,"/test/ABC.jpg",1
+,3,"/test/abc.jpg",1
diff --git a/Tests/Functional/Command/DataSet/sys_file_duplicates_mix_casesensitive_RESULT.csv b/Tests/Functional/Command/DataSet/sys_file_duplicates_mix_casesensitive_RESULT.csv
new file mode 100644
index 0000000..22650a8
--- /dev/null
+++ b/Tests/Functional/Command/DataSet/sys_file_duplicates_mix_casesensitive_RESULT.csv
@@ -0,0 +1,4 @@
+"sys_file",,,
+,"uid","identifier","storage"
+,2,"/test/ABC.jpg",1
+,3,"/test/abc.jpg",1
diff --git a/Tests/Functional/Command/UnduplicateCommandTest.php b/Tests/Functional/Command/UnduplicateCommandTest.php
index 2127b6a..aa4a371 100644
--- a/Tests/Functional/Command/UnduplicateCommandTest.php
+++ b/Tests/Functional/Command/UnduplicateCommandTest.php
@@ -50,6 +50,22 @@ class UnduplicateCommandTest extends FunctionalTestCase
         self::assertEquals(0, $result['status']);
     }
 
+    /**
+     * * abc.jpg
+     * * ABC.jpg
+     * * ABC.jpg
+     * should remove one ABC.jpg
+     */
+    #[Test] public function unduplicateCommandFixesDuplicatesWithMixCaseSensitives(): void
+    {
+        $this->importCSVDataSet(__DIR__ . '/DataSet/sys_file_duplicates_mix_casesensitive.csv');
+
+        $result = $this->executeConsoleCommand(self::BASE_COMMAND);
+
+        $this->assertCSVDataSet(__DIR__ . '/DataSet/sys_file_duplicates_mix_casesensitive_RESULT.csv');
+        self::assertEquals(0, $result['status']);
+    }
+
     #[Test] public function unduplicateCommandFixesDuplicatesWithReferences(): void
     {
         $this->importCSVDataSet(__DIR__ . '/DataSet/sys_file_duplicates_with_references.csv');