diff --git a/Frontmatter.Test/EquivalentKeyMergeTests.cs b/Frontmatter.Test/EquivalentKeyMergeTests.cs
new file mode 100644
index 0000000..83d2fc5
--- /dev/null
+++ b/Frontmatter.Test/EquivalentKeyMergeTests.cs
@@ -0,0 +1,68 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Frontmatter.Test;
+
+using System.Collections.Generic;
+using System.Linq;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Regression tests for keys that normalize to the same name, such as <c>related</c> and
+/// <c>page_related</c>. Each key used to be mapped to the other key's name, so the two were
+/// swapped rather than merged.
+/// </summary>
+[TestClass]
+public class EquivalentKeyMergeTests
+{
+	private static readonly string[] ListA = ["a"];
+	private static readonly string[] ListB = ["b"];
+
+	[TestMethod]
+	[DataRow(FrontmatterMergeStrategy.Aggressive, false)]
+	[DataRow(FrontmatterMergeStrategy.Aggressive, true)]
+	[DataRow(FrontmatterMergeStrategy.Maximum, false)]
+	[DataRow(FrontmatterMergeStrategy.Maximum, true)]
+	public void EquivalentListKeys_MergeUnderOneNameWithBothValues(FrontmatterMergeStrategy strategy, bool prefixedFirst)
+	{
+		Dictionary<string, object> frontmatter = prefixedFirst
+			? new() { ["page_related"] = ListB, ["related"] = ListA }
+			: new() { ["related"] = ListA, ["page_related"] = ListB };
+
+		Dictionary<string, object> result = PropertyMerger.MergeSimilarProperties(frontmatter, strategy);
+
+		Assert.HasCount(1, result);
+		Assert.IsTrue(result.TryGetValue("related", out object? related), "The unprefixed key should name the merged property");
+		CollectionAssert.AreEquivalent(new object[] { "a", "b" }, ((IEnumerable<object>)related).ToArray());
+	}
+
+	[TestMethod]
+	[DataRow(FrontmatterMergeStrategy.Aggressive, false)]
+	[DataRow(FrontmatterMergeStrategy.Aggressive, true)]
+	[DataRow(FrontmatterMergeStrategy.Maximum, false)]
+	[DataRow(FrontmatterMergeStrategy.Maximum, true)]
+	public void EquivalentScalarKeys_MergeIntoOneProperty(FrontmatterMergeStrategy strategy, bool prefixedFirst)
+	{
+		Dictionary<string, object> frontmatter = prefixedFirst
+			? new() { ["page_related"] = "y", ["related"] = "x" }
+			: new() { ["related"] = "x", ["page_related"] = "y" };
+
+		Dictionary<string, object> result = PropertyMerger.MergeSimilarProperties(frontmatter, strategy);
+
+		Assert.HasCount(1, result, "Two scalars that normalize alike should merge into one property");
+	}
+
+	[TestMethod]
+	public void CombineFrontmatter_Aggressive_MergesRelatedListsInsteadOfSwappingThem()
+	{
+		const string input = "---\nrelated:\n- a\npage_related:\n- b\n---\nBody\n";
+
+		string result = Frontmatter.CombineFrontmatter(input, FrontmatterNaming.AsIs, FrontmatterOrder.AsIs, FrontmatterMergeStrategy.Aggressive);
+
+		Dictionary<string, object>? frontmatter = Frontmatter.ExtractFrontmatter(result);
+		Assert.IsNotNull(frontmatter);
+		Assert.HasCount(1, frontmatter);
+		Assert.IsTrue(frontmatter.TryGetValue("related", out object? related), "The merged list should be written under related");
+		CollectionAssert.AreEquivalent(new object[] { "a", "b" }, ((IEnumerable<object>)related).ToArray());
+	}
+}
diff --git a/Frontmatter/PropertyMerger.cs b/Frontmatter/PropertyMerger.cs
index 323059a..baa50d2 100644
--- a/Frontmatter/PropertyMerger.cs
+++ b/Frontmatter/PropertyMerger.cs
@@ -3,6 +3,7 @@
 namespace ktsu.Frontmatter;
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 /// <summary>
@@ -40,9 +41,14 @@ internal static Dictionary<string, object> MergeSimilarProperties(Dictionary<str
 		// Group properties by their canonical names and merge values
 		foreach (IGrouping<string, string> group in propertyMappings.GroupBy(x => x.Value, x => x.Key))
 		{
-			string canonicalKey = group.Key;
 			List<string> originalKeys = [.. group];
 
+			// A key with nothing to merge with keeps its own name, so no key is ever renamed onto
+			// another key's name without the two actually being merged.
+			string canonicalKey = originalKeys.Count == 1 && frontmatter.ContainsKey(group.Key) && group.Key != originalKeys[0]
+				? originalKeys[0]
+				: group.Key;
+
 			MergePropertyGroup(frontmatter, mergedFrontmatter, canonicalKey, originalKeys);
 		}
 
@@ -203,19 +209,9 @@ private static string FindBasicCanonicalName(string key, string[] existingKeys)
 		}
 
 		// Look for exact matches after normalization
-		foreach (string existingKey in existingKeys)
+		if (TryFindEquivalenceClassName(key, normalizedKey, existingKeys, out string? className))
 		{
-			if (existingKey == key)
-			{
-				continue;
-			}
-
-			string normalizedExisting = NormalizePropertyName(existingKey);
-			if (string.Equals(normalizedKey, normalizedExisting, StringComparison.OrdinalIgnoreCase))
-			{
-				// If the existing key is a known property, use its canonical name
-				return PropertyMappings.All.TryGetValue(existingKey, out string? knownName) ? knownName : existingKey;
-			}
+			return className;
 		}
 
 		// Look for partial matches
@@ -235,6 +231,13 @@ private static string FindBasicCanonicalName(string key, string[] existingKeys)
 			if (normalizedKey.Contains(normalizedExisting2, StringComparison.OrdinalIgnoreCase) ||
 				normalizedExisting2.Contains(normalizedKey, StringComparison.OrdinalIgnoreCase))
 			{
+				// Both keys of a pair must agree on one name, or each would be renamed to the other.
+				string preferred = PreferredName([key, existingKey]);
+				if (preferred == key)
+				{
+					continue;
+				}
+
 				// If the existing key is a known property, use its canonical name
 				return PropertyMappings.All.TryGetValue(existingKey, out string? knownName2) ? knownName2 : existingKey;
 			}
@@ -243,6 +246,46 @@ private static string FindBasicCanonicalName(string key, string[] existingKeys)
 		return key;
 	}
 
+	/// <summary>
+	/// Finds the one name shared by every key that normalizes the same as <paramref name="key"/>.
+	/// </summary>
+	/// <param name="key">The key to analyze.</param>
+	/// <param name="normalizedKey">The normalized form of <paramref name="key"/>.</param>
+	/// <param name="existingKeys">All existing keys in the frontmatter.</param>
+	/// <param name="className">The name every key in the class maps to.</param>
+	/// <returns>True if at least one other key normalizes the same as <paramref name="key"/>.</returns>
+	private static bool TryFindEquivalenceClassName(string key, string normalizedKey, string[] existingKeys, [NotNullWhen(true)] out string? className)
+	{
+		List<string> equivalenceClass = [key];
+		equivalenceClass.AddRange(existingKeys.Where(existingKey => existingKey != key &&
+			string.Equals(normalizedKey, NormalizePropertyName(existingKey), StringComparison.OrdinalIgnoreCase)));
+
+		if (equivalenceClass.Count == 1)
+		{
+			className = null;
+			return false;
+		}
+
+		// Every key in the class must reach the same name, whichever of them is asking. Mapping each
+		// key to some other key instead sent related to page_related and page_related to related,
+		// which swapped their values rather than merging them.
+		string preferred = PreferredName(equivalenceClass);
+		className = PropertyMappings.All.TryGetValue(preferred, out string? knownName) ? knownName : preferred;
+		return true;
+	}
+
+	/// <summary>
+	/// Picks one name from a set of keys, independently of their order: a known property first,
+	/// then the shortest key, with any tie broken ordinally.
+	/// </summary>
+	/// <param name="keys">The keys to choose between.</param>
+	/// <returns>The preferred key.</returns>
+	private static string PreferredName(IEnumerable<string> keys) => keys
+		.OrderBy(k => PropertyMappings.All.ContainsKey(k) ? 0 : 1)
+		.ThenBy(k => k.Length)
+		.ThenBy(k => k, StringComparer.Ordinal)
+		.First();
+
 	/// <summary>
 	/// Attempts to find a canonical name for a property key using semantic analysis.
 	/// </summary>
@@ -258,6 +301,13 @@ private static string FindSemanticCanonicalName(string key, string[] existingKey
 			return basicMatch;
 		}
 
+		// The key is the name its equivalence class merges under, so it must not move elsewhere.
+		string normalizedKey = NormalizePropertyName(key);
+		if (normalizedKey.Length > 0 && TryFindEquivalenceClassName(key, normalizedKey, existingKeys, out _))
+		{
+			return key;
+		}
+
 		// Then try more aggressive matching using word similarity
 		string[] keyWords = PropertyNameNormalizer.NormalizeToWords(key);