diff --git a/smarttree/_check.py b/smarttree/_check.py
index d9ebeb9..cdb9e40 100644
--- a/smarttree/_check.py
+++ b/smarttree/_check.py
@@ -47,9 +47,13 @@ def check__params(
 
     if num_features is not None:
         _check__num_features(num_features)
+        param_name = "num_features"
+        _check__features_contain_duplicates(param_name, num_features)
 
     if cat_features is not None:
         _check__cat_features(cat_features)
+        param_name = "cat_features"
+        _check__features_contain_duplicates(param_name, cat_features)
 
     if rank_features is not None:
         _check__rank_features(rank_features)
@@ -212,6 +216,11 @@ def _check__rank_features(rank_features):
             )
 
 
+def _check__features_contain_duplicates(param_name, features):
+    if isinstance(features, list) and len(features) != len(set(features)):
+        raise ValueError(f"`{param_name}` contains duplicates.")
+
+
 def _check__hierarchy(hierarchy):
     common_message = (
         "`hierarchy` must be a dictionary"
diff --git a/tests/decision_tree/base/test__check_params.py b/tests/decision_tree/base/test__check_params.py
index b87c7b0..9043388 100644
--- a/tests/decision_tree/base/test__check_params.py
+++ b/tests/decision_tree/base/test__check_params.py
@@ -422,6 +422,26 @@ def test__check_params__rank_features(rank_features, expected_context):
         SmartDecisionTreeClassifier(rank_features=rank_features)
 
 
+@pytest.mark.parametrize(
+    ("params_to_set", "expected_context"),
+    [
+        ({"num_features": ["f1", "f2"]}, does_not_raise()),
+        (
+            {"num_features": ["f1", "f1"]},
+            pytest.raises(ValueError, match="`num_features` contains duplicates.")
+        ),
+        ({"cat_features": ["f1", "f2"]}, does_not_raise()),
+        (
+            {"cat_features": ["f1", "f1"]},
+            pytest.raises(ValueError, match="`cat_features` contains duplicates.")
+        ),
+    ],
+)
+def test__check_params__features_contain_duplicates(params_to_set, expected_context):
+    with expected_context:
+        SmartDecisionTreeClassifier(**params_to_set)
+
+
 @pytest.mark.parametrize(
     ("hierarchy", "expected_context"),
     [