diff --git a/feature_engine/_prediction/base_predictor.py b/feature_engine/_prediction/base_predictor.py
index 3fab9a3..3ab45be 100644
--- a/feature_engine/_prediction/base_predictor.py
+++ b/feature_engine/_prediction/base_predictor.py
@@ -270,7 +270,7 @@ class BaseTargetMeanEstimator(BaseEstimator):
 
         return X_tr
 
-    def _predict(self, X: pd.DataFrame) -> np.array:
+    def _predict(self, X: pd.DataFrame) -> np.ndarray:
         """
         Predict using the average of the target mean value across variables.
 
diff --git a/feature_engine/_prediction/target_mean_classifier.py b/feature_engine/_prediction/target_mean_classifier.py
index 484f5b8..3338f3f 100644
--- a/feature_engine/_prediction/target_mean_classifier.py
+++ b/feature_engine/_prediction/target_mean_classifier.py
@@ -125,7 +125,7 @@ class TargetMeanClassifier(BaseTargetMeanEstimator, ClassifierMixin):
 
         return super().fit(X, y)
 
-    def predict_proba(self, X: pd.DataFrame) -> np.array:
+    def predict_proba(self, X: pd.DataFrame) -> np.ndarray:
         """
         Predict class probabilities for X.
 
@@ -148,7 +148,7 @@ class TargetMeanClassifier(BaseTargetMeanEstimator, ClassifierMixin):
         prob = self._predict(X)
         return np.vstack([1 - prob, prob]).T
 
-    def predict_log_proba(self, X: pd.DataFrame) -> np.array:
+    def predict_log_proba(self, X: pd.DataFrame) -> np.ndarray:
         """
         Predict class log-probabilities for X.
 
@@ -167,7 +167,7 @@ class TargetMeanClassifier(BaseTargetMeanEstimator, ClassifierMixin):
         """
         return np.log(self.predict_proba(X))
 
-    def predict(self, X: pd.DataFrame) -> np.array:
+    def predict(self, X: pd.DataFrame) -> np.ndarray:
         """
         Predict class for X.
 
diff --git a/feature_engine/_prediction/target_mean_regressor.py b/feature_engine/_prediction/target_mean_regressor.py
index bd2d949..b220fc2 100644
--- a/feature_engine/_prediction/target_mean_regressor.py
+++ b/feature_engine/_prediction/target_mean_regressor.py
@@ -106,7 +106,7 @@ class TargetMeanRegressor(BaseTargetMeanEstimator, RegressorMixin):
 
         return super().fit(X, y)
 
-    def predict(self, X: pd.DataFrame) -> np.array:
+    def predict(self, X: pd.DataFrame) -> np.ndarray:
         """
         Predict using the average of the target mean value across variables.
 
diff --git a/feature_engine/variable_manipulation.py b/feature_engine/variable_manipulation.py
index 1608469..25ac9f1 100644
--- a/feature_engine/variable_manipulation.py
+++ b/feature_engine/variable_manipulation.py
@@ -27,11 +27,14 @@ def _check_input_parameter_variables(variables: Variables) -> Any:
     """
 
     msg = "variables should be a string, an int or a list of strings or integers."
+    msg_dupes = "the list contains duplicated variable names"
 
     if variables:
         if isinstance(variables, list):
             if not all(isinstance(i, (str, int)) for i in variables):
                 raise ValueError(msg)
+            if len(variables) != len(set(variables)):
+                raise ValueError(msg_dupes)
         else:
             if not isinstance(variables, (str, int)):
                 raise ValueError(msg)
@@ -250,7 +253,9 @@ def _find_or_check_datetime_variables(
 
 
 def _find_all_variables(
-    X: pd.DataFrame, variables: Variables = None, exclude_datetime: bool = False,
+    X: pd.DataFrame,
+    variables: Variables = None,
+    exclude_datetime: bool = False,
 ) -> List[Union[str, int]]:
     """
     If variables are None, captures all variables in the dataframe in a list.

diff --git a/tests/test_variable_manipulation.py b/tests/test_variable_manipulation.py
index 98606b6..c2294e2 100644
--- a/tests/test_variable_manipulation.py
+++ b/tests/test_variable_manipulation.py
@@ -12,30 +12,30 @@ from feature_engine.variable_manipulation import (
 )
 
 
-def test_check_input_parameter_variables():
-    vars_ls = ["var1", "var2", "var1"]
-    vars_int_ls = [0, 1, 2, 3]
-    vars_none = None
-    vars_str = "var1"
-    vars_int = 0
-    vars_tuple = ("var1", "var2")
-    vars_set = {"var1", "var2"}
-    vars_dict = {"var1": 1, "var2": 2}
-
-    assert _check_input_parameter_variables(vars_ls) == ["var1", "var2", "var1"]
-    assert _check_input_parameter_variables(vars_int_ls) == [0, 1, 2, 3]
-    assert _check_input_parameter_variables(vars_none) is None
-    assert _check_input_parameter_variables(vars_str) == "var1"
-    assert _check_input_parameter_variables(vars_int) == 0
-
+@pytest.mark.parametrize(
+    "_input_vars",
+    [
+        ("var1", "var2"),
+        {"var1": 1, "var2": 2},
+        ["var1", "var2", "var2", "var3"],
+        [0, 1, 1, 2],
+    ],
+)
+def test_check_input_parameter_variables_raises_errors(_input_vars):
     with pytest.raises(ValueError):
-        assert _check_input_parameter_variables(vars_tuple)
+        assert _check_input_parameter_variables(_input_vars)
 
-    with pytest.raises(ValueError):
-        assert _check_input_parameter_variables(vars_set)
 
-    with pytest.raises(ValueError):
-        assert _check_input_parameter_variables(vars_dict)
+@pytest.mark.parametrize(
+    "_input_vars",
+    [["var1", "var2", "var3"], [0, 1, 2, 3], "var1", ["var1"], 0, [0]],
+)
+def test_check_input_parameter_variables(_input_vars):
+    assert _check_input_parameter_variables(_input_vars) == _input_vars
+
+
+def test_check_input_parameter_variables_is_none():
+    assert _check_input_parameter_variables(None) is None
 
 
 def test_find_or_check_numerical_variables(df_vartypes, df_numeric_columns):
@@ -206,15 +206,12 @@ def test_find_or_check_datetime_variables(df_datetime):
         _find_or_check_datetime_variables(df_datetime, variables=None)
         == vars_convertible_to_dt
     )
-    assert (
-        _find_or_check_datetime_variables(
-            df_datetime[vars_convertible_to_dt].reindex(
-                columns=["date_obj1", "datetime_range", "date_obj2"]
-            ),
-            variables=None,
-        )
-        == ["date_obj1", "datetime_range", "date_obj2"]
-    )
+    assert _find_or_check_datetime_variables(
+        df_datetime[vars_convertible_to_dt].reindex(
+            columns=["date_obj1", "datetime_range", "date_obj2"]
+        ),
+        variables=None,
+    ) == ["date_obj1", "datetime_range", "date_obj2"]
 
     # when variables are specified
     assert _find_or_check_datetime_variables(df_datetime, var_dt_str) == [var_dt_str]
@@ -226,13 +223,10 @@ def test_find_or_check_datetime_variables(df_datetime):
         _find_or_check_datetime_variables(df_datetime, variables=vars_convertible_to_dt)
         == vars_convertible_to_dt
     )
-    assert (
-        _find_or_check_datetime_variables(
-            df_datetime.join(tz_time),
-            variables=None,
-        )
-        == vars_convertible_to_dt + ["time_objTZ"]
-    )
+    assert _find_or_check_datetime_variables(
+        df_datetime.join(tz_time),
+        variables=None,
+    ) == vars_convertible_to_dt + ["time_objTZ"]
 
     # datetime var cast as categorical
     df_datetime["date_obj1"] = df_datetime["date_obj1"].astype("category")

functions in variable manipulation should check that user did not enter duplicated variable names
@FYI @Morgan-Sell 

This one is important for your decision tree transformer
tests/test_variable_manipulation.py::test_check_input_parameter_variables_raises_errors[_input_vars2] tests/test_variable_manipulation.py::test_check_input_parameter_variables_raises_errors[_input_vars3]
tests/test_variable_manipulation.py::test_check_input_parameter_variables_raises_errors[_input_vars0] tests/test_variable_manipulation.py::test_check_input_parameter_variables_raises_errors[_input_vars1] tests/test_variable_manipulation.py::test_check_input_parameter_variables[_input_vars0] tests/test_variable_manipulation.py::test_check_input_parameter_variables[_input_vars1] tests/test_variable_manipulation.py::test_check_input_parameter_variables[var1] tests/test_variable_manipulation.py::test_check_input_parameter_variables[_input_vars3] tests/test_variable_manipulation.py::test_check_input_parameter_variables[0] tests/test_variable_manipulation.py::test_check_input_parameter_variables[_input_vars5] tests/test_variable_manipulation.py::test_check_input_parameter_variables_is_none tests/test_variable_manipulation.py::test_find_or_check_numerical_variables tests/test_variable_manipulation.py::test_find_or_check_categorical_variables tests/test_variable_manipulation.py::test_find_or_check_categorical_variables_when_numeric_is_cast_as_category_or_object[Age-category] tests/test_variable_manipulation.py::test_find_or_check_categorical_variables_when_numeric_is_cast_as_category_or_object[Age-O] tests/test_variable_manipulation.py::test_find_or_check_categorical_variables_when_numeric_is_cast_as_category_or_object[Marks-category] tests/test_variable_manipulation.py::test_find_or_check_categorical_variables_when_numeric_is_cast_as_category_or_object[Marks-O] tests/test_variable_manipulation.py::test_find_or_check_datetime_variables tests/test_variable_manipulation.py::test_find_or_check_datetime_variables_when_numeric_is_cast_as_category_or_object[Age-category] tests/test_variable_manipulation.py::test_find_or_check_datetime_variables_when_numeric_is_cast_as_category_or_object[Age-O] tests/test_variable_manipulation.py::test_find_all_variables tests/test_variable_manipulation.py::test_filter_out_variables_not_in_dataframe[df0-variables0-overlap0-not_in_col0] tests/test_variable_manipulation.py::test_filter_out_variables_not_in_dataframe[df1-variables1-overlap1-not_in_col1] tests/test_variable_manipulation.py::test_filter_out_variables_not_in_dataframe[df2-1-overlap2-7] tests/test_variable_manipulation.py::test_filter_out_variables_not_in_dataframe[df3-C-overlap3-G] tests/test_variable_manipulation.py::test_find_categorical_and_numerical_variables