diff --git a/src/obsidian_metadata/models/metadata.py b/src/obsidian_metadata/models/metadata.py
index 08fa312..2223a73 100644
--- a/src/obsidian_metadata/models/metadata.py
+++ b/src/obsidian_metadata/models/metadata.py
@@ -281,6 +281,7 @@ class Frontmatter:
         if key in self.dict and value not in self.dict[key]:
             if isinstance(value, list):
                 self.dict[key].extend(value)
+                self.dict[key] = list(sorted(set(self.dict[key])))
                 return True
 
             self.dict[key].append(value)
diff --git a/tests/alerts_test.py b/tests/alerts_test.py
index 130b42c..ee8a2de 100644
--- a/tests/alerts_test.py
+++ b/tests/alerts_test.py
@@ -107,7 +107,7 @@ def test_logging(capsys, tmp_path, verbosity, log_to_file) -> None:
     if verbosity >= 3:
         assert logging.is_trace() is True
         captured = capsys.readouterr()
-        assert captured.out == ""
+        assert not captured.out
 
         assert logging.is_trace("trace text") is True
         captured = capsys.readouterr()
@@ -128,7 +128,7 @@ def test_logging(capsys, tmp_path, verbosity, log_to_file) -> None:
     if verbosity >= 2:
         assert logging.is_debug() is True
         captured = capsys.readouterr()
-        assert captured.out == ""
+        assert not captured.out
 
         assert logging.is_debug("debug text") is True
         captured = capsys.readouterr()
@@ -149,7 +149,7 @@ def test_logging(capsys, tmp_path, verbosity, log_to_file) -> None:
     if verbosity >= 1:
         assert logging.is_info() is True
         captured = capsys.readouterr()
-        assert captured.out == ""
+        assert not captured.out
 
         assert logging.is_info("info text") is True
         captured = capsys.readouterr()
@@ -165,11 +165,11 @@ def test_logging(capsys, tmp_path, verbosity, log_to_file) -> None:
 
         log.info("This is Info logging")
         captured = capsys.readouterr()
-        assert captured.out == ""
+        assert not captured.out
 
     assert logging.is_default() is True
     captured = capsys.readouterr()
-    assert captured.out == ""
+    assert not captured.out
 
     assert logging.is_default("default text") is True
     captured = capsys.readouterr()
diff --git a/tests/application_test.py b/tests/application_test.py
index e9315d5..bcddf75 100644
--- a/tests/application_test.py
+++ b/tests/application_test.py
@@ -613,7 +613,7 @@ def test_transpose_metadata_1(test_application, mocker, capsys) -> None:
     assert "SUCCESS  | Transposed Inline Metadata to Frontmatter in 5 notes" in captured
 
 
-def test_transpose_metadata_2(test_application, mocker, capsys) -> None:
+def test_transpose_metadata_2(test_application, mocker) -> None:
     """Transpose metadata.
 
     GIVEN a test application
diff --git a/tests/metadata_test.py b/tests/metadata_test.py
index 2770d9f..7e43d1c 100644
--- a/tests/metadata_test.py
+++ b/tests/metadata_test.py
@@ -133,6 +133,100 @@ def test_frontmatter_create_4():
     assert frontmatter.dict == {}
 
 
+def test_frontmatter_add_1():
+    """Test frontmatter add() method.
+
+    GIVEN a Frontmatter object
+    WHEN the add() method is called with an existing key
+    THEN return False
+    """
+    frontmatter = Frontmatter(FRONTMATTER_CONTENT)
+
+    assert frontmatter.add("frontmatter_Key1") is False
+
+
+def test_frontmatter_add_2():
+    """Test frontmatter add() method.
+
+    GIVEN a Frontmatter object
+    WHEN the add() method is called with an existing key and existing value
+    THEN return False
+    """
+    frontmatter = Frontmatter(FRONTMATTER_CONTENT)
+    assert frontmatter.add("frontmatter_Key1", "frontmatter_Key1_value") is False
+
+
+def test_frontmatter_add_3():
+    """Test frontmatter add() method.
+
+    GIVEN a Frontmatter object
+    WHEN the add() method is called with a new key
+    THEN return True and add the key to the dict
+    """
+    frontmatter = Frontmatter(FRONTMATTER_CONTENT)
+    assert frontmatter.add("added_key") is True
+    assert "added_key" in frontmatter.dict
+
+
+def test_frontmatter_add_4():
+    """Test frontmatter add() method.
+
+    GIVEN a Frontmatter object
+    WHEN the add() method is called with a new key and a new value
+    THEN return True and add the key and the value to the dict
+    """
+    frontmatter = Frontmatter(FRONTMATTER_CONTENT)
+    assert frontmatter.add("added_key", "added_value") is True
+    assert frontmatter.dict["added_key"] == ["added_value"]
+
+
+def test_frontmatter_add_5():
+    """Test frontmatter add() method.
+
+    GIVEN a Frontmatter object
+    WHEN the add() method is called with an existing key and a new value
+    THEN return True and add the value to the dict
+    """
+    frontmatter = Frontmatter(FRONTMATTER_CONTENT)
+    assert frontmatter.add("frontmatter_Key1", "new_value") is True
+    assert frontmatter.dict["frontmatter_Key1"] == ["frontmatter_Key1_value", "new_value"]
+
+
+def test_frontmatter_add_6():
+    """Test frontmatter add() method.
+
+    GIVEN a Frontmatter object
+    WHEN the add() method is called with an existing key and a list of new values
+    THEN return True and add the values to the dict
+    """
+    frontmatter = Frontmatter(FRONTMATTER_CONTENT)
+    assert frontmatter.add("frontmatter_Key1", ["new_value", "new_value2"]) is True
+    assert frontmatter.dict["frontmatter_Key1"] == [
+        "frontmatter_Key1_value",
+        "new_value",
+        "new_value2",
+    ]
+
+
+def test_frontmatter_add_7():
+    """Test frontmatter add() method.
+
+    GIVEN a Frontmatter object
+    WHEN the add() method is called with an existing key and a list of values including an existing value
+    THEN return True and add the new values to the dict
+    """
+    frontmatter = Frontmatter(FRONTMATTER_CONTENT)
+    assert (
+        frontmatter.add("frontmatter_Key1", ["frontmatter_Key1_value", "new_value", "new_value2"])
+        is True
+    )
+    assert frontmatter.dict["frontmatter_Key1"] == [
+        "frontmatter_Key1_value",
+        "new_value",
+        "new_value2",
+    ]
+
+
 def test_frontmatter_contains_1():
     """Test frontmatter contains() method.
 
@@ -221,71 +315,6 @@ def test_frontmatter_contains_8():
     assert frontmatter.contains("key", r"_\d", is_regex=True) is False
 
 
-def test_frontmatter_add() -> None:
-    """Test frontmatter add."""
-    frontmatter = Frontmatter(FRONTMATTER_CONTENT)
-
-    assert frontmatter.add("frontmatter_Key1") is False
-    assert frontmatter.add("added_key") is True
-    assert frontmatter.dict == {
-        "added_key": [],
-        "frontmatter_Key1": ["frontmatter_Key1_value"],
-        "frontmatter_Key2": ["article", "note"],
-        "shared_key1": ["shared_key1_value"],
-        "tags": ["tag_1", "tag_2", "📅/tag_3"],
-    }
-
-    assert frontmatter.add("added_key", "added_value") is True
-    assert frontmatter.dict == {
-        "added_key": ["added_value"],
-        "frontmatter_Key1": ["frontmatter_Key1_value"],
-        "frontmatter_Key2": ["article", "note"],
-        "shared_key1": ["shared_key1_value"],
-        "tags": ["tag_1", "tag_2", "📅/tag_3"],
-    }
-
-    assert frontmatter.add("added_key", "added_value_2") is True
-    assert frontmatter.dict == {
-        "added_key": ["added_value", "added_value_2"],
-        "frontmatter_Key1": ["frontmatter_Key1_value"],
-        "frontmatter_Key2": ["article", "note"],
-        "shared_key1": ["shared_key1_value"],
-        "tags": ["tag_1", "tag_2", "📅/tag_3"],
-    }
-
-    assert frontmatter.add("added_key", ["added_value_3", "added_value_4"]) is True
-    assert frontmatter.dict == {
-        "added_key": ["added_value", "added_value_2", "added_value_3", "added_value_4"],
-        "frontmatter_Key1": ["frontmatter_Key1_value"],
-        "frontmatter_Key2": ["article", "note"],
-        "shared_key1": ["shared_key1_value"],
-        "tags": ["tag_1", "tag_2", "📅/tag_3"],
-    }
-
-    assert frontmatter.add("added_key2", ["added_value_1", "added_value_2"]) is True
-    assert frontmatter.dict == {
-        "added_key": ["added_value", "added_value_2", "added_value_3", "added_value_4"],
-        "added_key2": ["added_value_1", "added_value_2"],
-        "frontmatter_Key1": ["frontmatter_Key1_value"],
-        "frontmatter_Key2": ["article", "note"],
-        "shared_key1": ["shared_key1_value"],
-        "tags": ["tag_1", "tag_2", "📅/tag_3"],
-    }
-
-    assert frontmatter.add("added_key3", "added_value_1") is True
-    assert frontmatter.dict == {
-        "added_key": ["added_value", "added_value_2", "added_value_3", "added_value_4"],
-        "added_key2": ["added_value_1", "added_value_2"],
-        "added_key3": ["added_value_1"],
-        "frontmatter_Key1": ["frontmatter_Key1_value"],
-        "frontmatter_Key2": ["article", "note"],
-        "shared_key1": ["shared_key1_value"],
-        "tags": ["tag_1", "tag_2", "📅/tag_3"],
-    }
-
-    assert frontmatter.add("added_key3", "added_value_1") is False
-
-
 def test_frontmatter_rename() -> None:
     """Test frontmatter rename."""
     frontmatter = Frontmatter(FRONTMATTER_CONTENT)
diff --git a/tests/notes_test.py b/tests/notes_test.py
index 58700e0..737e49f 100644
--- a/tests/notes_test.py
+++ b/tests/notes_test.py
@@ -715,10 +715,9 @@ def test_transpose_metadata_4(sample_note):
         "intext_key": ["intext_value"],
         "key📅": ["📅_key_value"],
         "shared_key1": [
-            "shared_key1_value",
-            "shared_key1_value3",
             "shared_key1_value",
             "shared_key1_value2",
+            "shared_key1_value3",
         ],
         "shared_key2": ["shared_key2_value1", "shared_key2_value2"],
         "tags": [