don't mutate command on subsequent calls to Invoke extension

src/System.CommandLine.Tests/CommandTests.cs

@@ -336,18 +336,31 @@ public void When_multiple_options_are_configured_then_they_must_differ_by_name()
         }
 
         [Fact]
-        public void When_global_options_are_added_then_they_must_differ_from_local_options_by_name()
+        public void Global_options_may_be_added_with_aliases_that_conflict_with_local_options()
         {
             var command = new Command("the-command")
             {
                 new Option("--same")
             };
 
+            command
+                .Invoking(c => c.AddGlobalOption(new Option("--same")))
+                .Should()
+                .NotThrow<ArgumentException>();
+        }
+
+        [Fact]
+        public void Global_options_may_not_have_aliases_conflicting_with_other_global_option_aliases()
+        {
+            var command = new Command("the-command");
+
+            command.AddGlobalOption(new Option("--same"));
+
             command
                 .Invoking(c => c.AddGlobalOption(new Option("--same")))
                 .Should()
                 .Throw<ArgumentException>()
-                .And
+                .Which
                 .Message
                 .Should()
                 .Be("Alias '--same' is already in use.");

src/System.CommandLine/Builder/CommandLineBuilder.cs

@@ -17,6 +17,10 @@ public class CommandLineBuilder : CommandBuilder
         public CommandLineBuilder(Command rootCommand = null)
             : base(rootCommand ?? new RootCommand())
         {
+            if (rootCommand?.ImplicitParser != null)
+            {
+                throw new ArgumentException($"Command \"{rootCommand.Name}\" has already been configured.");
+            }
         }
 
         public bool EnableDirectives { get; set; } = true;
@@ -33,7 +37,7 @@ public Parser Build()
         {
             var rootCommand = Command;
 
-            return new Parser(
+            var parser = new Parser(
                 new CommandLineConfiguration(
                     new[] { rootCommand },
                     enablePosixBundling: EnablePosixBundling,
@@ -44,6 +48,10 @@ public Parser Build()
                                                        .Select(m => m.middleware)
                                                        .ToArray(),
                     helpBuilderFactory: HelpBuilderFactory));
+
+            Command.ImplicitParser = parser;
+
+            return parser;
         }
 
         internal void AddMiddleware(

src/System.CommandLine/Builder/CommandLineBuilderExtensions.cs

@@ -401,7 +401,6 @@ public static TBuilder UseHelpBuilder<TBuilder>(this TBuilder builder, Func<Bind
             return builder;
         }
 
-
         public static CommandLineBuilder UseTypoCorrections(
             this CommandLineBuilder builder, int maxLevenshteinDistance = 3)
         {

src/System.CommandLine/Command.cs

@@ -3,6 +3,7 @@
 
 using System.Collections;
 using System.Collections.Generic;
+using System.CommandLine.Builder;
 using System.CommandLine.Collections;
 using System.CommandLine.Invocation;
 using System.CommandLine.Parsing;
@@ -32,7 +33,6 @@ public Command(string name, string description = null) : base(new[] { name }, de
 
         public void AddGlobalOption(Option option)
         {
-            Children.ThrowIfAnyAliasIsInUse(option);
             _globalOptions.Add(option);
             Children.AddWithoutAliasCollisionCheck(option);
         }
@@ -68,5 +68,7 @@ private protected override void AddSymbol(Symbol symbol)
         IEnumerable<IArgument> ICommand.Arguments => Arguments;
 
         IEnumerable<IOption> ICommand.Options => Options;
+
+        internal Parser ImplicitParser { get; set; }
     }
 }

src/System.CommandLine/CommandExtensions.cs

@@ -42,11 +42,12 @@ public static class CommandExtensions
 
         private static InvocationPipeline GetInvocationPipeline(Command command, string[] args)
         {
-            var parser = new CommandLineBuilder(command)
-                         .UseDefaults()
-                         .Build();
+            var parser = command.ImplicitParser ??=
+                             new CommandLineBuilder(command)
+                                 .UseDefaults()
+                                 .Build();
 
-            ParseResult parseResult = parser.Parse(args);
+            var parseResult = parser.Parse(args);
 
             return new InvocationPipeline(parseResult);
         }