diff --git a/CodeBlocker.Test/DocCommentTests.cs b/CodeBlocker.Test/DocCommentTests.cs
index f802433..9c535fa 100644
--- a/CodeBlocker.Test/DocCommentTests.cs
+++ b/CodeBlocker.Test/DocCommentTests.cs
@@ -204,6 +204,37 @@ public void ValidationReportsADuplicateTag()
 		Assert.Contains("more than once", issues[0]);
 	}
 
+	[TestMethod]
+	public void ValidationReportsADuplicateUndeclaredTagOnce()
+	{
+		DocComment documentation = new();
+		documentation.Params.Add(new DocTag { Name = "y", Text = "First." });
+		documentation.Params.Add(new DocTag { Name = "y", Text = "Again." });
+		documentation.Params.Add(new DocTag { Name = "y", Text = "Third." });
+
+		IReadOnlyList<string> issues = documentation.Validate(["x"], []);
+
+		Assert.HasCount(4, issues);
+		Assert.AreEqual("<param name=\"y\"> does not match any declared parameter.", issues[0]);
+		Assert.AreEqual("<param name=\"y\"> is documented more than once.", issues[1]);
+		Assert.AreEqual("<param name=\"y\"> is documented more than once.", issues[2]);
+		Assert.AreEqual("The parameter 'x' has no <param> entry.", issues[3]);
+	}
+
+	[TestMethod]
+	public void ValidationReportsADuplicateUndeclaredTypeParameterTagOnce()
+	{
+		DocComment documentation = new();
+		documentation.TypeParams.Add(new DocTag { Name = "U", Text = "First." });
+		documentation.TypeParams.Add(new DocTag { Name = "U", Text = "Again." });
+
+		IReadOnlyList<string> issues = documentation.Validate([], []);
+
+		Assert.HasCount(2, issues);
+		Assert.AreEqual("<typeparam name=\"U\"> does not match any declared type parameter.", issues[0]);
+		Assert.AreEqual("<typeparam name=\"U\"> is documented more than once.", issues[1]);
+	}
+
 	[TestMethod]
 	public void ANamedTagWithEmptyTextIsStillWritten()
 	{
diff --git a/CodeBlocker/Templates/DocComment.cs b/CodeBlocker/Templates/DocComment.cs
index d82f857..505c3e4 100644
--- a/CodeBlocker/Templates/DocComment.cs
+++ b/CodeBlocker/Templates/DocComment.cs
@@ -167,8 +167,7 @@ void Check(Collection<DocTag> tags, HashSet<string> declared, string tagName, st
 				{
 					issues.Add($"<{tagName} name=\"{name}\"> is documented more than once.");
 				}
-
-				if (!declared.Contains(name))
+				else if (!declared.Contains(name))
 				{
 					issues.Add($"<{tagName} name=\"{name}\"> does not match any declared {what}.");
 				}