From cb1af46bb4d6052b38bca73d117f699456fca5de Mon Sep 17 00:00:00 2001
From: matt-edmondson <matthew.edmondson@gmail.com>
Date: Sun, 27 Sep 2026 07:25:08 +0000
Subject: [PATCH] [patch] Keep repeated items within a list when merging
 properties

MergeArrayValues collected every list into one HashSet, so it removed
duplicates inside a single list as well as across the lists being
merged. Because even singleton keys go through it, CombineFrontmatter
under the default Conservative strategy turned steps: [mix, wait, mix]
into [mix, wait] on a document with nothing to merge.

Lists are now appended in order, and an item is dropped only when an
earlier list in the same merge already supplied it.

Fixes #142

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KMqfUfzWj2Gmdz1EqYdnLe
---
 Frontmatter.Test/PropertyMergerTests.cs | 44 +++++++++++++++++++++++++
 Frontmatter/PropertyMerger.cs           | 18 +++++-----
 2 files changed, 54 insertions(+), 8 deletions(-)

diff --git a/Frontmatter.Test/PropertyMergerTests.cs b/Frontmatter.Test/PropertyMergerTests.cs
index 6d758e5..8a72a51 100644
--- a/Frontmatter.Test/PropertyMergerTests.cs
+++ b/Frontmatter.Test/PropertyMergerTests.cs
@@ -745,4 +745,48 @@ public void CombineFrontmatter_WithMaximumStrategy_MergesAllRelevantProperties()
 		// 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"]);
+	}
 }
diff --git a/Frontmatter/PropertyMerger.cs b/Frontmatter/PropertyMerger.cs
index 9eac550..70ab447 100644
--- a/Frontmatter/PropertyMerger.cs
+++ b/Frontmatter/PropertyMerger.cs
@@ -172,18 +172,20 @@ private static void MergeArrayValues(
 		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();