fix(commandbus): disallow having two #[CommandHandler] for the same command

src/Tempest/CommandBus/src/CommandBusConfig.php

@@ -17,8 +17,15 @@ public function __construct(
     ) {
     }
 
+    /**
+     * @throws CommandHandlerAlreadyExists
+     */
     public function addHandler(CommandHandler $commandHandler, string $commandName, MethodReflector $handler): self
     {
+        if (array_key_exists($commandName, $this->handlers)) {
+            throw new CommandHandlerAlreadyExists($commandName, new: $handler, existing: $this->handlers[$commandName]->handler);
+        }
+
         $this->handlers[$commandName] = $commandHandler
             ->setCommandName($commandName)
             ->setHandler($handler);

src/Tempest/CommandBus/src/CommandHandlerAlreadyExists.php

@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tempest\CommandBus;
+
+use Exception;
+use Tempest\Reflection\MethodReflector;
+
+final class CommandHandlerAlreadyExists extends Exception
+{
+    public function __construct(string $commandName, MethodReflector $new, MethodReflector $existing)
+    {
+        parent::__construct("Cannot add handler {$new->getShortName()}, {$existing->getShortName()} already handles {$commandName}.");
+    }
+}

src/Tempest/CommandBus/tests/Fixtures/CreateUserCommandHandler.php

@@ -18,4 +18,10 @@ public function __invoke(CreateUserCommand $command): void
         $this->firstName = $command->firstName;
         $this->lastName = $command->lastName;
     }
+
+    #[CommandHandler]
+    public function double(CreateUserCommand $command): void
+    {
+        // throws a CommandHandlerAlreadyExists exception since the command is already being handled by the __invoke method
+    }
 }

src/Tempest/CommandBus/tests/GenericCommandBusTest.php

@@ -8,6 +8,7 @@
 use Tempest\CommandBus\CommandBus;
 use Tempest\CommandBus\CommandBusConfig;
 use Tempest\CommandBus\CommandHandler;
+use Tempest\CommandBus\CommandHandlerAlreadyExists;
 use Tempest\CommandBus\CommandHandlerNotFound;
 use Tempest\CommandBus\GenericCommandBus;
 use Tempest\CommandBus\Tests\Fixtures\CreateUserCommand;
@@ -21,6 +22,8 @@
  */
 final class GenericCommandBusTest extends TestCase
 {
+    private CommandBusConfig $config;
+
     private CommandBus $commandBus;
 
     public function test_getting_command_handler_that_exists(): void
@@ -44,26 +47,41 @@ public function test_exception_is_thrown_when_command_handler_doesnt_exist(): vo
         $this->commandBus->dispatch($command);
     }
 
+    public function test_exception_is_thrown_when_command_handler_already_exist(): void
+    {
+        $createUserCommandHandlerClass = new ClassReflector(CreateUserCommandHandler::class);
+        $createUserCommandHandlerMethod = $createUserCommandHandlerClass->getMethod('__invoke');
+        $createUserCommandHandler = $createUserCommandHandlerMethod->getAttribute(CommandHandler::class);
+
+        $this->expectException(CommandHandlerAlreadyExists::class);
+
+        $this->config->addHandler(
+            commandHandler: $createUserCommandHandler,
+            commandName: CreateUserCommand::class,
+            handler: $createUserCommandHandlerClass->getMethod('double'),
+        );
+    }
+
     protected function setUp(): void
     {
         parent::setUp();
 
         // TODO: I'd like to make this easier to setup.
-        $config = new CommandBusConfig();
+        $this->config = new CommandBusConfig();
 
         $createUserCommandHandlerClass = new ClassReflector(CreateUserCommandHandler::class);
         $createUserCommandHandlerMethod = $createUserCommandHandlerClass->getMethod('__invoke');
         $createUserCommandHandler = $createUserCommandHandlerMethod->getAttribute(CommandHandler::class);
 
-        $config->addHandler(
+        $this->config->addHandler(
             commandHandler: $createUserCommandHandler,
             commandName: CreateUserCommand::class,
-            handler: $createUserCommandHandlerMethod
+            handler: $createUserCommandHandlerMethod,
         );
 
         $this->commandBus = new GenericCommandBus(
             container: new GenericContainer(),
-            commandBusConfig: $config
+            commandBusConfig: $this->config,
         );
     }
 }