diff --git a/tests/middleware_tests/test_sorting_entry_fields.py b/tests/middleware_tests/test_sorting_entry_fields.py
index b60ede9..ac178a3 100644
--- a/tests/middleware_tests/test_sorting_entry_fields.py
+++ b/tests/middleware_tests/test_sorting_entry_fields.py
@@ -1,27 +1,32 @@
 import pytest
 
 from bibtexparser import Library
+from bibtexparser import write_string
 from bibtexparser.middlewares.sorting_entry_fields import SortFieldsAlphabeticallyMiddleware
 from bibtexparser.middlewares.sorting_entry_fields import SortFieldsCustomMiddleware
 from bibtexparser.model import Entry
 from bibtexparser.model import Field
 
-TEST_ENTRY = Entry(
-    "article",
-    "Cesar2013",
-    [
-        Field("author", "Cesar, J.", 1),
-        Field("title", "A title", 2),
-        Field("note", "A note", 3),
-        Field("journal", "A journal", 3),
-        Field("year", "2013", 4),
-    ],
-)
+
+def _test_entry():
+    """Return a fresh entry so in-place middleware tests cannot affect each other."""
+    return Entry(
+        "article",
+        "Cesar2013",
+        [
+            Field("author", "Cesar, J.", 1),
+            Field("title", "A title", 2),
+            Field("note", "A note", 3),
+            Field("journal", "A journal", 3),
+            Field("year", "2013", 4),
+        ],
+    )
 
 
 @pytest.mark.parametrize("inplace", [True, False])
 def test_sort_alphabetically(inplace: bool):
-    library = Library(blocks=[TEST_ENTRY])
+    original_entry = _test_entry()
+    library = Library(blocks=[original_entry])
     m = SortFieldsAlphabeticallyMiddleware(allow_inplace_modification=inplace)
     transformed = m.transform(library)
     entry = transformed.entries[0]
@@ -34,16 +39,17 @@ def test_sort_alphabetically(inplace: bool):
     # Sanity Checks against unintended external effects
     assert len(transformed.blocks) == 1
     if inplace:
-        assert entry is TEST_ENTRY
+        assert entry is original_entry
     else:
-        assert entry is not TEST_ENTRY
+        assert entry is not original_entry
 
 
 @pytest.mark.parametrize("inplace", [True, False])
 def test_sort_custom_order_case_insensitive(inplace: bool):
     custom_order = ("author", "yEar", "title", "Journal")  # assess case insensitive
 
-    library = Library(blocks=[TEST_ENTRY])
+    original_entry = _test_entry()
+    library = Library(blocks=[original_entry])
     # case insensitivity is default
     m = SortFieldsCustomMiddleware(order=custom_order, allow_inplace_modification=inplace)
     transformed = m.transform(library)
@@ -57,28 +63,60 @@ def test_sort_custom_order_case_insensitive(inplace: bool):
     # Sanity Checks against unintended external effects
     assert len(transformed.blocks) == 1
     if inplace:
-        assert entry is TEST_ENTRY
+        assert entry is original_entry
     else:
-        assert entry is not TEST_ENTRY
+        assert entry is not original_entry
 
 
 def test_sort_order_custom_case_sensitive():
     custom_order = ("author", "yEar", "title", "Journal")  # assess case sensitive
 
-    library = Library(blocks=[TEST_ENTRY])
+    library = Library(blocks=[_test_entry()])
     m = SortFieldsCustomMiddleware(order=custom_order, case_sensitive=True)
     transformed = m.transform(library)
     entry = transformed.entries[0]
     assert entry.fields[0].key == "author"
     assert entry.fields[1].key == "title"
 
-    # Unspecified fields (due to case sensitivity) are appended
-    assert entry.fields[2].key == "year"
-
-    # Unspecified fields (due to case sensitivity) are appended
+    # Unspecified fields are appended in their original relative order.
+    assert entry.fields[2].key == "note"
     assert entry.fields[3].key == "journal"
-
-    # Unspecified fields are appended
-    assert entry.fields[4].key == "note"
+    assert entry.fields[4].key == "year"
 
     assert len(transformed.blocks) == 1
+
+
+def test_custom_order_applies_during_public_write():
+    """The public writer applies custom ordering without mutating the source library."""
+    original_entry = _test_entry()
+    library = Library(blocks=[original_entry])
+    middleware = SortFieldsCustomMiddleware(
+        order=("year", "author"), allow_inplace_modification=False
+    )
+
+    written = write_string(library, prepend_middleware=[middleware])
+    written_field_keys = [
+        line.split("=", maxsplit=1)[0].strip() for line in written.splitlines() if "=" in line
+    ]
+
+    assert written_field_keys == ["year", "author", "title", "note", "journal"]
+    assert [field.key for field in original_entry.fields] == [
+        "author",
+        "title",
+        "note",
+        "journal",
+        "year",
+    ]
+
+
+@pytest.mark.parametrize(
+    "order",
+    [
+        pytest.param(("title", "title"), id="identical"),
+        pytest.param(("title", "TITLE"), id="case-insensitive"),
+    ],
+)
+def test_custom_order_rejects_duplicate_keys(order):
+    """Ambiguous custom orders fail early, including case-only duplicates by default."""
+    with pytest.raises(ValueError, match="title"):
+        SortFieldsCustomMiddleware(order=order)