"Might indicate a bad software design"

There is a bit of code that reads:
```
future = self.futures.get(key)
if future:
if self.todo[key] != value:
# I don't think this is likely to happen. I'd like to know about it if
# it does because that might indicate a bad software design.
future = tasklets.Future()
future.set_exception(
RuntimeError(
"Key has already been set in this batch: {}".format(key)
)
)
```
Ok, we're letting you know. We have Python 3.9 code running in Appengine and are seeing "Key has already been set in this batch" often enough that I felt compelled to check this issue.
The expression in our code that triggers this is
`ndb.get_multi(event.guestlists)` and the stack trace below that is
```
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/_options.py", line 89, in wrapper return wrapped(*pass_args, **kwargs)
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/utils.py", line 153, in positional_wrapper return wrapped(*args, **kwds) File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/model.py", line 6427, in get_multi return [future.result() for future in futures]
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/model.py", line 6427, in return [future.result() for future in futures]
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/tasklets.py", line 214, in result self.check_success()
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/tasklets.py", line 161, in check_success raise self._exception
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/tasklets.py", line 334, in _advance_tasklet yielded = self.generator.throw(type(error), error, traceback)
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/key.py", line 894, in get entity_pb = yield _datastore_api.lookup(self._key, _options)
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/tasklets.py", line 334, in _advance_tasklet yielded = self.generator.throw(type(error), error, traceback)
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/_datastore_api.py", line 150, in lookup lock = yield _cache.global_lock_for_read(cache_key, result)
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/tasklets.py", line 334, in _advance_tasklet yielded = self.generator.throw(type(error), error, traceback)
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/_cache.py", line 607, in global_lock_for_read lock_acquired = yield global_compare_and_swap(key, lock, expires=_LOCK_TIME)
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/tasklets.py", line 334, in _advance_tasklet yielded = self.generator.throw(type(error), error, traceback)
File "/layers/google.python.pip/pip/lib/python3.9/site-packages/google/cloud/ndb/_cache.py", line 185, in wrapper result = yield function(key, *args, **kwargs) RuntimeError: Key has already been set in this batch: b'NDB30\n\x15\x12\x13evite-services-prod\x12,\n\nEventModel\x1a\x1e0175E4IF2RRZTMEEIEPMHZZKQTIFYA\x12\x18\n\rGuestListItem\x10\x80\x80\xe4\xe0\x88\xa6\xb4\x0b'
```

I tried calling `ndb.get_multi()`, explicitly passing in duplicate keys, and it didn't trigger this issue.