fix(folder-rename): prevent duplicate year suffixes in series folder names

Use regex to strip all trailing year suffixes before adding the canonical
one, preventing duplication like 'Show (2021) (2021) (2021)'.

- Add regex pattern (\s*\(\d{4}\))+\s*$ to remove all existing year suffixes
- Ensure idempotent behavior across multiple folder rename runs
- Add 7 unit tests covering the bug cases and edge scenarios

Fixes: 86 Eighty Six (2021) (2021)..., Alma-chan (2025) (2025)...

tests/unit/test_folder_rename_service.py

@@ -75,6 +75,84 @@ class TestComputeExpectedFolderName:
         result = _compute_expected_folder_name("A / B", "2021")
         assert result == "A B (2021)"
 
+    def test_does_not_duplicate_year(self) -> None:
+        result = _compute_expected_folder_name("86 Eighty Six (2021)", "2021")
+        assert result == "86 Eighty Six (2021)"
+        assert result.count("(2021)") == 1
+
+    def test_removes_duplicate_year_suffixes_bug_86_eighty_six(self) -> None:
+        """Test the bug fix for duplicate year suffixes.
+        
+        Issue: "86 Eighty Six (2021) (2021) (2021) (2021) (2021)"
+        should become "86 Eighty Six (2021)"
+        """
+        result = _compute_expected_folder_name(
+            "86 Eighty Six (2021) (2021) (2021) (2021) (2021)", "2021"
+        )
+        assert result == "86 Eighty Six (2021)"
+        assert result.count("(2021)") == 1
+
+    def test_removes_duplicate_year_suffixes_alma_chan(self) -> None:
+        """Test the bug fix for duplicate year suffixes with long title.
+        
+        Issue: "Alma-chan Wants to Be a Family! (2025) (2025) (2025) (2025) (2025)"
+        should become "Alma-chan Wants to Be a Family! (2025)"
+        """
+        result = _compute_expected_folder_name(
+            "Alma-chan Wants to Be a Family! (2025) (2025) (2025) (2025) (2025)",
+            "2025",
+        )
+        assert result == "Alma-chan Wants to Be a Family! (2025)"
+        assert result.count("(2025)") == 1
+
+    def test_removes_duplicate_year_suffixes_bogus_skill(self) -> None:
+        """Test the bug fix for duplicate year suffixes with very long title.
+        
+        Issue: Long title with duplicated years should be cleaned.
+        """
+        result = _compute_expected_folder_name(
+            "Bogus Skill Fruitmaster About That Time I Became Able to Eat "
+            "Unlimited Numbers of Skill Fruits (That Kill You) (2025) (2025)",
+            "2025",
+        )
+        assert "(2025)" in result
+        assert result.count("(2025)") == 1
+
+    def test_removes_multiple_different_year_suffixes(self) -> None:
+        """Test that old duplicate years are removed and new one added."""
+        result = _compute_expected_folder_name(
+            "Series (2020) (2020) (2020)", "2021"
+        )
+        assert result == "Series (2021)"
+        assert "(2020)" not in result
+        assert result.count("(2021)") == 1
+
+    def test_handles_whitespace_with_duplicate_years(self) -> None:
+        """Test that extra whitespace is removed along with duplicate years."""
+        result = _compute_expected_folder_name(
+            "Series (2021)  (2021)  (2021)  ", "2021"
+        )
+        assert result == "Series (2021)"
+        assert result.count("(2021)") == 1
+        assert not result.endswith(" ")
+
+    def test_idempotent_multiple_calls(self) -> None:
+        """Test that calling the function multiple times produces the same result."""
+        title = "86 Eighty Six (2021) (2021) (2021)"
+        year = "2021"
+
+        # First call
+        result1 = _compute_expected_folder_name(title, year)
+        # Second call with the result
+        result2 = _compute_expected_folder_name(result1, year)
+        # Third call with the result
+        result3 = _compute_expected_folder_name(result2, year)
+
+        # All results should be identical
+        assert result1 == result2 == result3
+        assert result1 == "86 Eighty Six (2021)"
+        assert result1.count("(2021)") == 1
+
 
 class TestIsSeriesBeingDownloaded:
     """Tests for _is_series_being_downloaded."""