diff --git a/src/pymgrid/envs/base/base.py b/src/pymgrid/envs/base/base.py
index 8df52e9c..fcc3a2d0 100644
--- a/src/pymgrid/envs/base/base.py
+++ b/src/pymgrid/envs/base/base.py
@@ -1,5 +1,6 @@
 import numpy as np
 import pandas as pd
+import warnings
 
 from collections import OrderedDict
 from gym import Env
@@ -122,7 +123,19 @@ def _validate_observation_keys(self, keys):
         if net_load_pos.size:
             keys[[0, net_load_pos.item()]] = keys[[net_load_pos.item(), 0]]
 
-        return keys.tolist()
+        unique_keys, dupe_keys = [], []
+
+        for k in keys:
+            if k in unique_keys:
+                dupe_keys.append(k)
+                continue
+
+            unique_keys.append(k)
+
+        if dupe_keys:
+            warnings.warn(f'Found duplicated keys, will be dropped:\n\t{dupe_keys}')
+
+        return unique_keys
 
     @abstractmethod
     def _get_action_space(self, remove_redundant_actions=False):
@@ -133,7 +146,12 @@ def _get_observation_space(self):
 
         state_series = self.state_series()
 
-        observation_keys = self.observation_keys or state_series.index.get_level_values(-1)
+        if self.observation_keys is None or len(self.observation_keys) == 0:
+            observation_keys = state_series.index.get_level_values(-1)
+        else:
+            observation_keys = pd.Index(self.observation_keys)
+
+        observation_keys = observation_keys.drop_duplicates()
 
         if 'net_load' in observation_keys:
             obs_space['general'] = Tuple([Box(low=-np.inf, high=1, shape=(1, ), dtype=np.float64)])