Keep repeated items within a list when merging properties

Frontmatter.Test/PropertyMergerTests.cs

@@ -745,4 +745,48 @@
 		// Tags variants should be merged
 		Assert.IsTrue(extractedFrontmatter.ContainsKey("tags"), "Result should contain key 'tags'");
 	}
+
+	[TestMethod]
+	public void MergeSimilarProperties_SingleListWithRepeatedItems_KeepsEveryItem()
+	{
+		Dictionary<string, object> frontmatter = new()
+		{
+			{ "steps", new List<object> { "mix", "wait", "mix" } },
+			{ "scores", new List<object> { "1", "1", "2" } }
+		};
+
+		Dictionary<string, object> result = PropertyMerger.MergeSimilarProperties(frontmatter, FrontmatterMergeStrategy.Conservative);
+
+		CollectionAssert.AreEqual(new object[] { "mix", "wait", "mix" }, (System.Collections.ICollection)result["steps"]);
+		CollectionAssert.AreEqual(new object[] { "1", "1", "2" }, (System.Collections.ICollection)result["scores"]);
+	}
+
+	[TestMethod]
+	public void MergeSimilarProperties_MergedListsRepeatWithinOneList_KeepsTheRepeatsAndDropsOnlyCrossListDuplicates()
+	{
+		Dictionary<string, object> frontmatter = new()
+		{
+			{ "tags", new List<object> { "a", "a", "c" } },
+			{ "tag", new List<object> { "b", "c", "b" } }
+		};
+
+		Dictionary<string, object> result = PropertyMerger.MergeSimilarProperties(frontmatter, FrontmatterMergeStrategy.Conservative);
+
+		Assert.HasCount(1, result);
+		CollectionAssert.AreEqual(new object[] { "a", "a", "c", "b", "b" }, (System.Collections.ICollection)result["tags"]);
+	}
+
+	[TestMethod]
+	public void CombineFrontmatter_SingleBlockWithRepeatedListItems_RoundTripsTheLists()
+	{
+		string nl = Environment.NewLine;
+		string input = $"---{nl}steps: [mix, wait, mix]{nl}scores: [1, 1, 2]{nl}---{nl}Body{nl}";
+
+		string result = Frontmatter.CombineFrontmatter(input, FrontmatterNaming.AsIs, FrontmatterOrder.AsIs, FrontmatterMergeStrategy.Conservative);
+		Dictionary<string, object>? frontmatter = Frontmatter.ExtractFrontmatter(result);
+
+		Assert.IsNotNull(frontmatter);
+		CollectionAssert.AreEqual(new object[] { "mix", "wait", "mix" }, (System.Collections.ICollection)frontmatter["steps"]);
+		CollectionAssert.AreEqual(new object[] { "1", "1", "2" }, (System.Collections.ICollection)frontmatter["scores"]);
+	}
 }

Frontmatter/PropertyMerger.cs

@@ -103,7 +103,7 @@
 
 		return !isKnownMapping && !hasExactMatch && !isInSameCategory
 			? key
 			: PropertyMappings.All.TryGetValue(canonicalName, out string? knownName) ? knownName : canonicalName;
 	}
 
 	private static bool IsInSameCategory(string key, string canonicalName)
@@ -172,18 +172,20 @@
 		string canonicalKey,
 		List<string> originalKeys)
 	{
-		HashSet<object> mergedList = [];
+		// Lists are appended in order. An item is dropped only when an earlier list being merged already
+		// supplied it, never because it repeats within its own list: a list such as recipe steps or
+		// scores can legitimately hold the same value more than once.
+		List<object> mergedList = [];
+		HashSet<object> fromEarlierLists = [];
 		foreach (string key in originalKeys)
 		{
-			object value = source[key];
-			if (value is IList<object> list)
+			if (source[key] is not IList<object> items)
 			{
-				mergedList.UnionWith(list);
-			}
-			else if (value is object[] array)
-			{
-				mergedList.UnionWith(array);
+				continue;
 			}
+
+			mergedList.AddRange(items.Where(item => !fromEarlierLists.Contains(item)));
+			fromEarlierLists.UnionWith(items);
 		}
 
 		target[canonicalKey] = mergedList.ToArray();
@@ -195,7 +197,7 @@
 	/// <param name="key">The key to analyze.</param>
 	/// <param name="existingKeys">All existing keys in the frontmatter.</param>
 	/// <returns>The canonical name for the key.</returns>
 	private static string FindBasicCanonicalName(string key, string[] existingKeys)
 	{
 		// First check if it's a known property
 		if (PropertyMappings.All.TryGetValue(key, out string? canonicalName))