diff --git a/src/python/jw/pkg/lib/App.py b/src/python/jw/pkg/lib/App.py
index 2df96c67..123aad2f 100644
--- a/src/python/jw/pkg/lib/App.py
+++ b/src/python/jw/pkg/lib/App.py
@@ -7,13 +7,15 @@ import sys
 import warnings
 
 from argparse import ArgumentDefaultsHelpFormatter, ArgumentParser, Namespace
-from typing import TYPE_CHECKING, Any, cast, override
+from typing import TYPE_CHECKING, Any, NamedTuple, cast, override
 
 from .AsyncRunner import AsyncRunner
+from .Cmd import AbstractCmd
 from .log import (
     DEBUG,
     ERR,
     NOTICE,
+    WARNING,
     LogFlag,
     log,
     log_m,
@@ -43,6 +45,11 @@ def _get_current_event_loop() -> asyncio.AbstractEventLoop | None:
         except (RuntimeError, DeprecationWarning):
             return None
 
+class _SubCommand(NamedTuple):
+
+    cmd: AbstractCmd
+    parser: ArgumentParser
+
 class App:  # export
 
     def _add_arguments(self, parser: ArgumentParser) -> None:
@@ -105,8 +112,6 @@ class App:  # export
         eloop: asyncio.AbstractEventLoop | None = None,
     ) -> None:
 
-        from .Cmd import AbstractCmd
-
         self.__args: Namespace | None = None
         self.__cmdline: str | None = None
         self.__description = description
@@ -171,23 +176,25 @@ class App:  # export
             if not cmds:
                 return
 
-            class SubCommand:
-
-                def __init__(self, cmd: AbstractCmd, parser: Any):
-                    self.cmd = cmd
-                    self.parser = parser
-
             title = 'Available subcommands'
             if isinstance(parent, AbstractCmd):
                 title += ' of ' + parent.name
             subparsers = parser.add_subparsers(
                 title = title, metavar = '', dest = 'command'
             )
-            scs: dict[str, SubCommand] = {}
+            scs: dict[str, _SubCommand] = {}
             for cmd in cmds:
                 cmd.set_parent(parent)
-                scs[cmd.name] = SubCommand(cmd, add_cmd_to_parser(cmd, subparsers))
+                if cmd.name in scs:
+                    log(WARNING, f'Duplicate subcommand name: {cmd.name}')
+                scs[cmd.name] = _SubCommand(cmd, add_cmd_to_parser(cmd, subparsers))
                 for alias in cmd.aliases:
+                    if alias != cmd.name and alias in scs:
+                        log(
+                            WARNING,
+                            f'Subcommand alias "{alias}" of "{cmd.name}" '
+                            'collides with an earlier subcommand',
+                        )
                     scs[alias] = scs[cmd.name]
             if all:
                 seen: set[int] = set()
@@ -198,14 +205,16 @@ class App:  # export
                             sc.cmd, sc.parser, sc.cmd.children, all = all
                         )
                 return
+            # -- Re-parse the command line to find the invoked subcommand.
+            # This works because every level below uses dest = 'command',
+            # so each pass descends one level further into the command
+            # tree.
             args, _ = self.__parser.parse_known_args(argv)
             cmd_name = getattr(args, 'command', None)
             if cmd_name in scs:
                 sc = scs[cmd_name]
                 add_cmds_to_parser(sc.cmd, sc.parser, sc.cmd.children, all = all)
 
-        from .Cmd import AbstractCmd
-
         cmdline = sys.argv if argv is None else argv
         if argv is None:
             argv = sys.argv[1:]