[FLAML Crash] [Classification] ValueError: Categorical categories must be unique

Hey, thanks for the great system.

I am experiencing a crash with a specific dataset. I get the following error when I fit FLAML on the [Higgs](https://www.openml.org/search?type=data&status=active&id=23512) dataset, a binary classification dataset used in the FLAML paper:
```
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/automl.py", line 1524, in fit
self._search()
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/automl.py", line 2009, in _search
self._search_sequential()
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/automl.py", line 1825, in _search_sequential
use_ray=False,
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/tune/tune.py", line 382, in run
result = training_function(trial_to_run.config)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/automl.py", line 240, in _compute_with_config_base
self.fit_kwargs,
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/ml.py", line 323, in compute_estimator
log_training_metric=log_training_metric, fit_kwargs=fit_kwargs)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/ml.py", line 196, in get_test_loss
estimator.fit(X_train, y_train, budget, **fit_kwargs)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/model.py", line 731, in fit
X_train = self._preprocess(X_train)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/model.py", line 689, in _preprocess
lambda x:
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/frame.py", line 8740, in apply
return op.apply()
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/apply.py", line 688, in apply
return self.apply_standard()
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/apply.py", line 812, in apply_standard
results, res_index = self.apply_series_generator()
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/apply.py", line 828, in apply_series_generator
results[i] = self.f(v)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/flaml/model.py", line 692, in
for c in x.cat.categories]))
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/accessor.py", line 93, in f
return self._delegate_method(name, *args, **kwargs)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/arrays/categorical.py", line 2631, in _delegate_method
res = method(*args, **kwargs)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/arrays/categorical.py", line 1053, in rename_categories
cat.categories = new_categories
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/arrays/categorical.py", line 733, in categories
new_dtype = CategoricalDtype(categories, ordered=self.ordered)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/dtypes/dtypes.py", line 183, in __init__
self._finalize(categories, ordered, fastpath=False)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/dtypes/dtypes.py", line 337, in _finalize
categories = self.validate_categories(categories, fastpath=fastpath)
File "/home/mossad/anaconda3/envs/kgpip/lib/python3.7/site-packages/pandas/core/dtypes/dtypes.py", line 540, in validate_categories
raise ValueError("Categorical categories must be unique")
ValueError: Categorical categories must be unique
```

Here is my script:
```
df = pd.read_csv('higgs.csv')
X, y = df.drop('class', axis=1), df['class']
X_train, X_test, y_train, y_test = train_test_split(X, y, random_state=123)
automl_model = AutoML()
automl_model.fit(X_train, y_train, task='classification',
time_budget=300,
retrain_full='budget',
verbose=0, metric='macro_f1')
```

Your feedback is appreciated.