diff --git a/scripts/performance/benchmark_utils.py b/scripts/performance/benchmark_utils.py
index da48ae372d81..333eed1e129e 100644
--- a/scripts/performance/benchmark_utils.py
+++ b/scripts/performance/benchmark_utils.py
@@ -23,8 +23,8 @@ def summarize(script, result_dir, summary_dir):
     with open(os.path.join(summary_dir, 'summary.txt'), 'wb') as f:
         subprocess.check_call(summarize_args, stdout=f)
     with open(os.path.join(summary_dir, 'summary.json'), 'wb') as f:
-        summarize_args.extend(['--output-format', 'json'])
-        subprocess.check_call(summarize_args, stdout=f)
+        json_args = summarize_args + ['--output-format', 'json']
+        subprocess.check_call(json_args, stdout=f)
 
 
 def _get_s3transfer_performance_script(script_name):
diff --git a/tests/unit/test_benchmark_utils.py b/tests/unit/test_benchmark_utils.py
new file mode 100644
index 000000000000..aaabee02d5f9
--- /dev/null
+++ b/tests/unit/test_benchmark_utils.py
@@ -0,0 +1,122 @@
+# Copyright 2024 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"). You
+# may not use this file except in compliance with the License. A copy of
+# the License is located at
+#
+#     http://aws.amazon.com/apache2.0/
+#
+# or in the "license" file accompanying this file. This file is
+# distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
+# ANY KIND, either express or implied. See the License for the specific
+# language governing permissions and limitations under the License.
+import os
+import sys
+import tempfile
+import unittest
+from unittest import mock
+
+# Add the scripts directory to the path so we can import benchmark_utils
+scripts_dir = os.path.join(
+    os.path.dirname(os.path.dirname(os.path.dirname(__file__))),
+    'scripts',
+    'performance'
+)
+sys.path.insert(0, scripts_dir)
+
+import benchmark_utils  # noqa: E402
+
+
+class TestSummarize(unittest.TestCase):
+    """Test cases for the summarize function."""
+
+    @mock.patch('benchmark_utils.subprocess.check_call')
+    def test_summarize_does_not_mutate_args(self, mock_check_call):
+        """Verify that summarize() does not mutate the argument list.
+
+        The function should use a new list for the JSON output call
+        rather than extending the original argument list.
+        """
+        with tempfile.TemporaryDirectory() as tmpdir:
+            result_dir = os.path.join(tmpdir, 'results')
+            summary_dir = os.path.join(tmpdir, 'summary')
+            os.makedirs(result_dir)
+            os.makedirs(summary_dir)
+
+            # Create a dummy CSV file in result_dir
+            test_file = os.path.join(result_dir, 'test.csv')
+            with open(test_file, 'w') as f:
+                f.write('dummy,data\n')
+
+            script = '/path/to/summarize'
+
+            # Capture the arguments passed to each check_call invocation
+            call_args_list = []
+
+            def capture_args(args, **kwargs):
+                # Make a copy to capture the state at call time
+                call_args_list.append(list(args))
+
+            mock_check_call.side_effect = capture_args
+
+            benchmark_utils.summarize(script, result_dir, summary_dir)
+
+            # Verify we had exactly 2 calls
+            self.assertEqual(len(call_args_list), 2)
+
+            # First call should have: [script, test_file]
+            first_call_args = call_args_list[0]
+            self.assertEqual(first_call_args[0], script)
+            self.assertIn(test_file, first_call_args)
+            self.assertNotIn('--output-format', first_call_args)
+
+            # Second call should have: [script, test_file, --output-format, json]  # noqa: E501
+            second_call_args = call_args_list[1]
+            self.assertEqual(second_call_args[0], script)
+            self.assertIn(test_file, second_call_args)
+            self.assertIn('--output-format', second_call_args)
+            self.assertIn('json', second_call_args)
+
+    @mock.patch('benchmark_utils.subprocess.check_call')
+    def test_summarize_multiple_calls_are_independent(self, mock_check_call):
+        """Verify that multiple calls to summarize() are independent.
+
+        This tests the scenario from issue #10121 where calling summarize()
+        multiple times could result in duplicated CLI arguments if the
+        original list was mutated.
+        """
+        with tempfile.TemporaryDirectory() as tmpdir:
+            result_dir = os.path.join(tmpdir, 'results')
+            summary_dir = os.path.join(tmpdir, 'summary')
+            os.makedirs(result_dir)
+            os.makedirs(summary_dir)
+
+            test_file = os.path.join(result_dir, 'test.csv')
+            with open(test_file, 'w') as f:
+                f.write('dummy,data\n')
+
+            script = '/path/to/summarize'
+            call_args_list = []
+
+            def capture_args(args, **kwargs):
+                call_args_list.append(list(args))
+
+            mock_check_call.side_effect = capture_args
+
+            # Call summarize twice
+            benchmark_utils.summarize(script, result_dir, summary_dir)
+            benchmark_utils.summarize(script, result_dir, summary_dir)
+
+            # Should have 4 calls total (2 per summarize invocation)
+            self.assertEqual(len(call_args_list), 4)
+
+            # Check that neither of the JSON calls have duplicated flags
+            for i, args in enumerate(call_args_list):
+                output_format_count = args.count('--output-format')
+                # Each call should have at most 1 --output-format flag
+                msg = f"Call {i} has {output_format_count} flags: {args}"
+                self.assertLessEqual(output_format_count, 1, msg)
+
+
+if __name__ == '__main__':
+    unittest.main()