Prevent the registration of duplicate subparser

- Addition of a duplicate subparser will result in an exception under
  Python 3.11

Fixes: #64

src/calmjs/runtime.py

@@ -487,6 +487,14 @@ def to_module_attr(ep):
             return '%s:%s' % (ep.module_name, '.'.join(ep.attrs))
 
         def register(name, runtime, entry_point):
+            if name in subparsers:
+                logger.critical(
+                    "another subparser was already registered as %r; "
+                    "aborting the registration of its associated runtime",
+                    name
+                )
+                return
+
             subparser = commands.add_parser(
                 name, help=inst.description,
             )

src/calmjs/tests/test_runtime.py

@@ -1883,9 +1883,14 @@ def cleanup():
 
         # EXPLOSION
         msg = stderr.getvalue()
-        self.assertIn("CRITICAL", msg)
         self.assertIn(
             "Runtime instance has been used or initialized improperly.", msg)
+        # The actual duplicated runtime may be non-deterministic as the
+        # order of which the registration are run that will result in
+        # the duplicate is derived from an unordered set.
+        self.assertIn(
+            "CRITICAL calmjs.runtime another subparser was already registered "
+            "as", msg)
         # Naisu Bakuretsu - Megumin.
 
     @unittest.skipIf(currentframe() is None, 'stack frame not supported')