Run a flag's handler once however many times it was given

TrayApp.Test/CommandLineParserTests.cs

@@ -136,6 +136,32 @@
 		Assert.IsTrue(ran);
 	}
 
+	[TestMethod]
+	public void TryParse_WithAFlagGivenUnderSeveralAliases_RunsItsHandlerOnce()
+	{
+		int calls = 0;
+		AppOption display = AppOption.Flag(["-d", "--display"], "Keep the display lit.", () => calls++);
+
+		Assert.IsTrue(CommandLineParser.TryParse(["-d", "--display", "-d"], [display], out CommandLineOptions options, out _));
+		Assert.IsTrue(TrayAppBuilder.TryApplyOptions(options, out _));
+
+		// A handler that toggles would otherwise be flipped back by the second spelling.
+		Assert.AreEqual(1, calls);
+		Assert.AreEqual(1, options.Matches.Count);
+	}
+
+	[TestMethod]
+	public void TryParse_WithAValueOptionGivenTwice_AppliesBothInOrder()
+	{
+		List<string> seen = [];
+		AppOption reason = AppOption.Value(["-r", "--reason"], "text", "Why.", seen.Add);
+
+		Assert.IsTrue(CommandLineParser.TryParse(["-r", "first", "--reason", "second"], [reason], out CommandLineOptions options, out _));
+		Assert.IsTrue(TrayAppBuilder.TryApplyOptions(options, out _));
+
+		CollectionAssert.AreEqual(new[] { "first", "second" }, seen);
+	}
+
 	[TestMethod]
 	public void TryParse_WithAnApplicationValueOption_TakesTheNextArgument()
 	{

TrayApp/Cli/AppOption.cs

@@ -80,8 +80,10 @@ public static AppOption Flag(IReadOnlyList<string> aliases, string description,
 	/// <param name="valueName">The placeholder shown in <c>--help</c>, such as <c>text</c>.</param>
 	/// <param name="description">The one-line description shown in <c>--help</c>.</param>
 	/// <param name="onValue">
-	/// Called with the value. Throw <see cref="FormatException"/> or <see cref="ArgumentException"/> from
-	/// here to reject a value: the message becomes the usage error, and the tool exits 2 without starting.
+	/// Called with the value, once for each time the option was given and in the order given, so the last
+	/// one wins for a handler that stores it. Throw <see cref="FormatException"/> or
+	/// <see cref="ArgumentException"/> from here to reject a value: the message becomes the usage error, and
+	/// the tool exits 2 without starting.
 	/// </param>
 	/// <returns>The option.</returns>
 	/// <exception cref="ArgumentNullException">Any argument is <see langword="null"/>.</exception>

TrayApp/Cli/CommandLineParser.cs

@@ -168,7 +168,13 @@ private static bool TryReadAppOption(
 			return false;
 		}
 
-		state.Matches.Add(new AppOptionMatch(matched, value));
+		// A switch runs its handler once however many times it was given, under whichever aliases, so a
+		// handler that toggles is not undone by "-d --display". A value option keeps every occurrence.
+		if (matched.TakesValue || !state.Matches.Exists(match => ReferenceEquals(match.Option, matched)))
+		{
+			state.Matches.Add(new AppOptionMatch(matched, value));
+		}
+
 		error = string.Empty;
 		return true;
 	}

TrayApp/TrayAppBuilder.cs

@@ -244,7 +244,7 @@ public TrayAppBuilder Separator()
 	/// </summary>
 	/// <param name="aliases">The spellings that select it, such as <c>["-d", "--display"]</c>.</param>
 	/// <param name="description">The one-line description shown in <c>--help</c>.</param>
-	/// <param name="onSet">Called once when the switch is present.</param>
+	/// <param name="onSet">Called once when the switch is present, however many times it was given.</param>
 	/// <returns>The builder, for chaining.</returns>
 	/// <exception cref="ArgumentNullException">Any argument is <see langword="null"/>.</exception>
 	public TrayAppBuilder Flag(IReadOnlyList<string> aliases, string description, Action onSet)
@@ -260,8 +260,9 @@ public TrayAppBuilder Flag(IReadOnlyList<string> aliases, string description, Ac
 	/// <param name="valueName">The placeholder shown in <c>--help</c>, such as <c>text</c>.</param>
 	/// <param name="description">The one-line description shown in <c>--help</c>.</param>
 	/// <param name="onValue">
-	/// Called with the value. Throw <see cref="FormatException"/> or <see cref="ArgumentException"/> to
-	/// reject it: the message becomes the usage error and the tool exits 2.
+	/// Called with the value, once for each time the option was given and in the order given. Throw
+	/// <see cref="FormatException"/> or <see cref="ArgumentException"/> to reject it: the message becomes
+	/// the usage error and the tool exits 2.
 	/// </param>
 	/// <returns>The builder, for chaining.</returns>
 	/// <exception cref="ArgumentNullException">Any argument is <see langword="null"/>.</exception>