Abort on ambiguous subcommand names

Sources/ArgumentParser/Parsing/CommandParser.swift

@@ -41,6 +41,8 @@ struct CommandParser {
       self.commandTree = try Tree(root: rootCommand)
     } catch Tree<ParsableCommand.Type>.InitializationError.recursiveSubcommand(let command) {
       fatalError("The ParsableCommand \"\(command)\" can't have itself as its own subcommand.")
+    } catch Tree<ParsableCommand.Type>.InitializationError.matchingCommandNames(let command, let commandName) {
+      fatalError("Ambiguous subcommand name \"\(commandName)\" encountered in ParsableCommand \"\(command)\"")
     } catch {
       fatalError("Unexpected error: \(error).")
     }

Sources/ArgumentParser/Utilities/Tree.swift

@@ -90,7 +90,11 @@ extension Tree where Element == ParsableCommand.Type {
 
   convenience init(root command: ParsableCommand.Type) throws {
     self.init(command)
+    var subcommandNames = Set<String>()
     for subcommand in command.configuration.subcommands {
+      if !subcommandNames.insert(subcommand._commandName).inserted {
+        throw InitializationError.matchingCommandNames(command, subcommand._commandName)
+      }
       if subcommand == command {
         throw InitializationError.recursiveSubcommand(subcommand)
       }
@@ -100,5 +104,6 @@ extension Tree where Element == ParsableCommand.Type {
 
   enum InitializationError: Error {
     case recursiveSubcommand(ParsableCommand.Type)
+    case matchingCommandNames(ParsableCommand.Type, String)
   }
 }

Tests/ArgumentParserUnitTests/TreeTests.swift

@@ -70,3 +70,35 @@ extension TreeTests {
     XCTAssertThrowsError(try Tree(root: Root.asCommand))
   }
 }
+
+extension TreeTests {
+  struct RootWithSubs: ParsableCommand {
+    static let configuration = CommandConfiguration(subcommands: [Sub.self, SubTwo.self])
+
+    struct Sub: ParsableCommand {
+    }
+
+    struct SubTwo: ParsableCommand {
+      static let configuration = CommandConfiguration(commandName: "sub")
+    }
+  }
+
+  struct RootWithNestedSubs: ParsableCommand {
+    static let configuration = CommandConfiguration(subcommands: [Sub.self])
+
+    struct Sub: ParsableCommand {
+      static let configuration = CommandConfiguration(subcommands: [Nested.self, NestedTwo.self])
+      struct Nested: ParsableCommand {
+      }
+
+      struct NestedTwo: ParsableCommand {
+        static let configuration = CommandConfiguration(commandName: "nested")
+      }
+    }
+  }
+
+  func testInitializationWithMatchingCommandNames() {
+    XCTAssertThrowsError(try Tree(root: RootWithSubs.asCommand))
+    XCTAssertThrowsError(try Tree(root: RootWithNestedSubs.asCommand))
+  }
+}