diff --git a/test_collections/matter/sdk_tests/support/python_testing/sdk_python_tests.py b/test_collections/matter/sdk_tests/support/python_testing/sdk_python_tests.py
index 0fa51066..5d9aef11 100644
--- a/test_collections/matter/sdk_tests/support/python_testing/sdk_python_tests.py
+++ b/test_collections/matter/sdk_tests/support/python_testing/sdk_python_tests.py
@@ -131,7 +131,10 @@ def __parse_python_tests(
             and collection_type != CollectionType.MANDATORY
         ):
             suites[SuiteType.NO_COMMISSIONING].add_test_case(test_case)
-        elif collection_type != CollectionType.MANDATORY:
+        elif (
+            python_test_type != PythonTestType.MANDATORY
+            and collection_type != CollectionType.MANDATORY
+        ):
             suites[SuiteType.LEGACY].add_test_case(test_case)
 
     return [s for s in list(suites.values()) if len(s.test_cases) != 0]
diff --git a/test_collections/matter/sdk_tests/support/tests/python_tests/test_python_script/mandatory_tests_info.json b/test_collections/matter/sdk_tests/support/tests/python_tests/test_python_script/mandatory_tests_info.json
new file mode 100644
index 00000000..a7cf30c3
--- /dev/null
+++ b/test_collections/matter/sdk_tests/support/tests/python_tests/test_python_script/mandatory_tests_info.json
@@ -0,0 +1,35 @@
+{
+    "sdk_sha": "aabbccddeeff11223344556677889900",
+    "tests": [
+        {
+            "class_name": "TC_SameClass",
+            "desc": "TC TC_SameClass description 1",
+            "function": "test_TC_SC_1_1",
+            "path": "sdk/TC_SameClass",
+            "pics": [],
+            "steps": [
+                {
+                    "description": "Step 1",
+                    "expectation": "",
+                    "is_commissioning": false,
+                    "test_plan_number": 1
+                }
+            ]
+        },
+        {
+            "class_name": "TC_DeviceConformance",
+            "desc": "TC_IDM_10_2 description",
+            "function": "test_TC_IDM_10_2",
+            "path": "sdk/TC_DeviceConformance",
+            "pics": [],
+            "steps": [
+                {
+                    "description": "Run entire test",
+                    "expectation": "",
+                    "is_commissioning": false,
+                    "test_plan_number": 1
+                }
+            ]
+        }
+    ]
+}
diff --git a/test_collections/matter/sdk_tests/support/tests/python_tests/test_sdk_python_collection.py b/test_collections/matter/sdk_tests/support/tests/python_tests/test_sdk_python_collection.py
index f85e544c..23c55b28 100644
--- a/test_collections/matter/sdk_tests/support/tests/python_tests/test_sdk_python_collection.py
+++ b/test_collections/matter/sdk_tests/support/tests/python_tests/test_sdk_python_collection.py
@@ -26,7 +26,10 @@
     PythonCaseDeclaration,
     PythonCollectionDeclaration,
 )
-from ...python_testing.sdk_python_tests import sdk_python_test_collection
+from ...python_testing.sdk_python_tests import (
+    sdk_mandatory_python_test_collection,
+    sdk_python_test_collection,
+)
 
 
 @pytest.fixture
@@ -42,6 +45,23 @@ def python_test_collection() -> PythonCollectionDeclaration:
         return sdk_python_test_collection(folder, tests_file_path=test_sdk_python_path)
 
 
+def _load_collections_from(
+    tests_file_path: Path,
+) -> tuple[PythonCollectionDeclaration, PythonCollectionDeclaration]:
+    with mock.patch.object(Path, "exists", return_value=True), mock.patch(
+        "test_collections.matter.sdk_tests.support.models.sdk_test_folder.open",
+        new=mock.mock_open(read_data="unit-test-python-version"),
+    ):
+        folder = SDKTestFolder(path=tests_file_path, filename_pattern="TC_*")
+        non_mandatory = sdk_python_test_collection(
+            folder, tests_file_path=tests_file_path
+        )
+        mandatory = sdk_mandatory_python_test_collection(
+            folder, tests_file_path=tests_file_path
+        )
+        return non_mandatory, mandatory
+
+
 def test_sdk_python_test_collection(
     python_test_collection: PythonCollectionDeclaration,
 ) -> None:
@@ -64,3 +84,27 @@ def test_automated_suite(python_test_collection: PythonCollectionDeclaration) ->
         type_count[test_case.test_type] += 1
 
     assert type_count[MatterTestType.AUTOMATED] == expected_automated_test_cases
+
+
+def test_mandatory_test_not_duplicated_across_collections() -> None:
+    """Regression test for issue #1087.
+
+    A mandatory test case (e.g. TC_IDM_10_2) must be present in the
+    "Mandatory SDK Python Tests" collection only, and must not also be
+    duplicated into the "Old script format" (LEGACY) suite of the
+    non-mandatory "SDK Python Tests" collection.
+    """
+    tests_file_path = (
+        Path(__file__).parent / "test_python_script/mandatory_tests_info.json"
+    )
+    non_mandatory_collection, mandatory_collection = _load_collections_from(
+        tests_file_path
+    )
+
+    for suite in non_mandatory_collection.test_suites.values():
+        assert "TC_IDM_10_2" not in suite.test_cases
+
+    mandatory_suite_name = "Python Testing Suite - Mandatories"
+    assert mandatory_suite_name in mandatory_collection.test_suites.keys()
+    mandatory_suite = mandatory_collection.test_suites[mandatory_suite_name]
+    assert "TC_IDM_10_2" in mandatory_suite.test_cases