diff --git a/README.md b/README.md
index 5a5753b..192c881 100644
--- a/README.md
+++ b/README.md
@@ -148,7 +148,9 @@ with the matching package references in the tool, not in the library:
 <PackageReference Include="ktsu.Essentials.SerializationProviders.Json" />
 ```
 
-Only toggles given a `persistAs` key are remembered, and only the tray reads them: a command line run
+Only toggles given a `persistAs` key are remembered, and each needs a key of its own: `Toggle` throws
+`ArgumentException` for a blank key or one another toggle already uses, since two toggles sharing a key
+would both be restored to whichever was clicked last. Only the tray reads them: a command line run
 says what it wants, so letting a remembered setting override an argument would be surprising. Within
 the tray, what was typed this time beats what the tray was left set to last time.
 
diff --git a/TrayApp.Test/TrayAppBuilderTests.cs b/TrayApp.Test/TrayAppBuilderTests.cs
index 44607aa..e020eda 100644
--- a/TrayApp.Test/TrayAppBuilderTests.cs
+++ b/TrayApp.Test/TrayAppBuilderTests.cs
@@ -72,6 +72,46 @@ public void Flag_WithAnAliasAnotherOptionTook_Throws()
 		Assert.AreEqual(1, builder.Build().Options.Count);
 	}
 
+	[TestMethod]
+	public void Toggle_WithAPersistAsKeyAnotherToggleTook_Throws()
+	{
+		// Both toggles would read and write one remembered value, so the next launch restores both to it.
+		TrayAppBuilder builder = TrayAppBuilder.Create("demo")
+			.Toggle("Keep awake", () => false, _ => { }, persistAs: "awake");
+
+		ArgumentException ex = Assert.ThrowsExactly<ArgumentException>(
+			() => builder.Toggle("Keep display on", () => false, _ => { }, persistAs: "awake"));
+
+		Assert.Contains("'awake'", ex.Message, StringComparison.Ordinal);
+		Assert.AreEqual("persistAs", ex.ParamName);
+		Assert.HasCount(1, builder.Build().Toggles);
+	}
+
+	[TestMethod]
+	public void Toggle_WithPersistAsKeysThatDifferOnlyInCase_KeepsBoth()
+	{
+		// The store compares keys ordinally, so these are two remembered values.
+		TrayAppDefinition app = TrayAppBuilder.Create("demo")
+			.Toggle("Keep awake", () => false, _ => { }, persistAs: "awake")
+			.Toggle("Keep display on", () => false, _ => { }, persistAs: "Awake")
+			.Toggle("Not remembered", () => false, _ => { })
+			.Toggle("Not remembered either", () => false, _ => { })
+			.Build();
+
+		Assert.HasCount(4, app.Toggles);
+	}
+
+	[TestMethod]
+	[DataRow("")]
+	[DataRow("   ")]
+	public void Toggle_WithABlankPersistAsKey_Throws(string key)
+	{
+		ArgumentException ex = Assert.ThrowsExactly<ArgumentException>(
+			() => TrayAppBuilder.Create("demo").Toggle("Keep awake", () => false, _ => { }, persistAs: key));
+
+		Assert.AreEqual("persistAs", ex.ParamName);
+	}
+
 	[TestMethod]
 	public void Build_CollectsItemsTogglesAndOptions()
 	{
diff --git a/TrayApp/TrayAppBuilder.cs b/TrayApp/TrayAppBuilder.cs
index 73cf5ca..fdbc14c 100644
--- a/TrayApp/TrayAppBuilder.cs
+++ b/TrayApp/TrayAppBuilder.cs
@@ -205,9 +205,14 @@ public TrayAppBuilder ActiveWhen(Func<bool> predicate)
 	/// <param name="persistAs">
 	/// The name to remember this toggle under between runs, or <see langword="null"/> not to remember it.
 	/// Only meaningful alongside <see cref="Preferences(IPersistenceProvider{string}, string, TimeSpan?)"/>.
+	/// Each remembered toggle needs a key of its own, compared ordinally: two toggles sharing one would read and
+	/// write a single remembered value, and the next launch would restore both to it.
 	/// </param>
 	/// <returns>The builder, for chaining.</returns>
 	/// <exception cref="ArgumentNullException"><paramref name="label"/>, <paramref name="getState"/>, or <paramref name="setState"/> is <see langword="null"/>.</exception>
+	/// <exception cref="ArgumentException">
+	/// <paramref name="persistAs"/> is empty or white space, or another toggle already uses it.
+	/// </exception>
 	public TrayAppBuilder Toggle(
 		string label,
 		Func<bool> getState,
@@ -215,6 +220,19 @@ public TrayAppBuilder Toggle(
 		Func<bool>? isEnabled = null,
 		string? persistAs = null)
 	{
+		if (persistAs is not null)
+		{
+			if (string.IsNullOrWhiteSpace(persistAs))
+			{
+				throw new ArgumentException("A preference key cannot be empty or white space.", nameof(persistAs));
+			}
+
+			if (toggles.Any(existing => string.Equals(existing.PreferenceKey, persistAs, StringComparison.Ordinal)))
+			{
+				throw new ArgumentException($"'{persistAs}' is already remembered by another toggle.", nameof(persistAs));
+			}
+		}
+
 		TrayToggleItem toggle = new(label, getState, setState, isEnabled, persistAs);
 		items.Add(toggle);
 		toggles.Add(toggle);