BUG: DataFrame.melt gives unexpected result when column "value" already exists

doc/source/whatsnew/v1.1.0.rst

@@ -1105,6 +1105,7 @@ Reshaping
 - Fixed bug in :func:`melt` where melting MultiIndex columns with ``col_level`` > 0 would raise a ``KeyError`` on ``id_vars`` (:issue:`34129`)
 - Bug in :meth:`Series.where` with an empty Series and empty ``cond`` having non-bool dtype (:issue:`34592`)
 - Fixed regression where :meth:`DataFrame.apply` would raise ``ValueError`` for elements whth ``S`` dtype (:issue:`34529`)
+- Issue warning if value column name already exists when using :meth:`DataFrame.melt` (:issue:`34731`)
 
 Sparse
 ^^^^^^

pandas/core/reshape/melt.py

@@ -1,5 +1,6 @@
 import re
 from typing import TYPE_CHECKING, List, cast
+import warnings
 
 import numpy as np
 
@@ -40,6 +41,14 @@ def melt(
     else:
         cols = list(frame.columns)
 
+    if value_name in frame.columns:
+        warnings.warn(
+            'The value column name "%s" conflicts with an existing '
+            "column in the dataframe." % (value_name),
+            DeprecationWarning,
+            stacklevel=3,
+        )
+
     if id_vars is not None:
         if not is_list_like(id_vars):
             id_vars = [id_vars]

pandas/tests/reshape/test_melt.py

@@ -1014,3 +1014,12 @@ def test_col_substring_of_stubname(self):
         )
         result = pd.wide_to_long(wide_df, stubnames="PA", i=["node_id", "A"], j="time")
         tm.assert_frame_equal(result, expected)
+
+    def test_warn_of_column_name_value(self):
+        # GH34731
+        # raise a warning if the resultant value column name matches
+        # a name in the dataframe already (default name is "value")
+        df = pd.DataFrame({"col": list("ABC"), "value": range(10, 16, 2)})
+
+        with tm.assert_produces_warning(DeprecationWarning):
+            dfm = df.melt(id_vars="value")  # noqa F841