diff --git a/SourceGeneratorToolkit.Test/DiagnosticCatalogTests.cs b/SourceGeneratorToolkit.Test/DiagnosticCatalogTests.cs
index 102489c..65e293a 100644
--- a/SourceGeneratorToolkit.Test/DiagnosticCatalogTests.cs
+++ b/SourceGeneratorToolkit.Test/DiagnosticCatalogTests.cs
@@ -66,6 +66,37 @@ public void DescriptorsAreRecordedInAllocationOrder()
 		CollectionAssert.AreEqual(ExpectedAllocationOrder, ids);
 	}
 
+	[TestMethod]
+	public void ReusingANumberAcrossSeveritiesThrows()
+	{
+		DiagnosticCatalog catalog = new("X", "c");
+		catalog.Warning(1, "a", "a");
+
+		ArgumentException exception = Assert.ThrowsExactly<ArgumentException>(() => catalog.Error(1, "b", "b"));
+
+		Assert.AreEqual("number", exception.ParamName);
+		Assert.AreEqual(1, catalog.Descriptors.Count);
+	}
+
+	[TestMethod]
+	public void ReusingANumberWithTheSameSeverityThrows()
+	{
+		DiagnosticCatalog catalog = new("X", "c");
+		catalog.Warning(1, "a", "a");
+
+		Assert.ThrowsExactly<ArgumentException>(() => catalog.Warning(1, "a", "a"));
+	}
+
+	[TestMethod]
+	public void TheSameNumberIsAllowedInSeparateCatalogues()
+	{
+		DiagnosticCatalog first = new("X", "c");
+		DiagnosticCatalog second = new("Y", "c");
+
+		Assert.AreEqual("X001", first.Warning(1, "a", "a").Id);
+		Assert.AreEqual("Y001", second.Warning(1, "a", "a").Id);
+	}
+
 	[TestMethod]
 	public void DescriptorsIsEmptyBeforeAnythingIsAllocated()
 	{
diff --git a/SourceGeneratorToolkit/DiagnosticCatalog.cs b/SourceGeneratorToolkit/DiagnosticCatalog.cs
index be738f5..357fb0b 100644
--- a/SourceGeneratorToolkit/DiagnosticCatalog.cs
+++ b/SourceGeneratorToolkit/DiagnosticCatalog.cs
@@ -24,6 +24,7 @@ namespace ktsu.SourceGeneratorToolkit;
 public sealed class DiagnosticCatalog(string idPrefix, string category)
 {
 	private readonly List<DiagnosticDescriptor> descriptors = [];
+	private readonly HashSet<int> numbers = [];
 
 	/// <summary>Gets the category every descriptor in this catalogue is reported under.</summary>
 	public string Category { get; } = category;
@@ -40,6 +41,9 @@ public sealed class DiagnosticCatalog(string idPrefix, string category)
 	/// <param name="title">The diagnostic title.</param>
 	/// <param name="messageFormat">The message format string.</param>
 	/// <returns>The descriptor, also recorded in <see cref="Descriptors"/>.</returns>
+	/// <exception cref="ArgumentException">
+	/// <paramref name="number"/> has already been allocated from this catalogue.
+	/// </exception>
 	public DiagnosticDescriptor Warning(int number, string title, string messageFormat) =>
 		Add(number, title, messageFormat, DiagnosticSeverity.Warning);
 
@@ -50,13 +54,25 @@ public DiagnosticDescriptor Warning(int number, string title, string messageForm
 	/// <param name="title">The diagnostic title.</param>
 	/// <param name="messageFormat">The message format string.</param>
 	/// <returns>The descriptor, also recorded in <see cref="Descriptors"/>.</returns>
+	/// <exception cref="ArgumentException">
+	/// <paramref name="number"/> has already been allocated from this catalogue.
+	/// </exception>
 	public DiagnosticDescriptor Error(int number, string title, string messageFormat) =>
 		Add(number, title, messageFormat, DiagnosticSeverity.Error);
 
 	private DiagnosticDescriptor Add(int number, string title, string messageFormat, DiagnosticSeverity severity)
 	{
+		string id = idPrefix + number.ToString("D3", CultureInfo.InvariantCulture);
+
+		// Two descriptors under one identifier would share every .editorconfig severity override and
+		// #pragma suppression meant for either, so a reused number is a mistake to catch here.
+		if (!numbers.Add(number))
+		{
+			throw new ArgumentException($"Diagnostic number {number} is already allocated as {id}.", nameof(number));
+		}
+
 		DiagnosticDescriptor descriptor = new(
-			id: idPrefix + number.ToString("D3", CultureInfo.InvariantCulture),
+			id: id,
 			title: title,
 			messageFormat: messageFormat,
 			category: Category,