[awaisdev5765]

Fixes #976

Problem

Duplicate subcommand names were silently ignored: firstChild(withName:) resolves to the first match, so with two subcommands sharing a commandName, only the first was invokable while the second remained listed in --help but could never run. This is the behavior reported in #976.

Fix

Tree.init(root:) now throws a new InitializationError.duplicateSubcommand when a sibling subcommand's name (or alias) collides with an already-registered name/alias at that level. CommandParser.init catches it via configurationFailure, following the established pattern for recursiveSubcommand and aliasMatchingCommand.

The check also catches alias collisions across sibling subcommands, which would otherwise shadow each other in the same way.

Example failure:

Error: The command "duplicate" can't be used as a subcommand more than once.

Testing

• initializationWithDuplicateSubcommands: two subcommands named "duplicate" → throws (the issue's exact scenario)
• initializationWithConflictingAliases: two subcommands sharing an alias → throws
• initializationWithDistinctSubcommandsSucceeds: distinct name and alias → no false positive
• Full suite: 575 tests in 57 suites, all passing