BUG: improve error message for duplicate columns in pd.melt (GH61475)

[mriganshu16]

Closes #61475

Problem Description

When calling pd.melt() on a DataFrame with duplicate column names,
the previous error message was generic and didn't identify which column
caused the problem:

x = pd.DataFrame([[1, 2, 3], [3, 4, 5]], columns=["A", "A", "B"])
pd.melt(x, id_vars=["A"], value_vars=["B"])
# Before: ValueError: id_vars cannot contain duplicate columns.
# After:  ValueError: id_vars contains columns with duplicate labels
#         in the DataFrame: ['A']. Please rename these columns before melting.

Changes

• pandas/core/reshape/melt.py: replaced the generic check with one
  that identifies and names the specific duplicate column(s)
• pandas/tests/reshape/test_melt.py: updated existing test to match
  new message; added a second test to verify the column name appears
  in the error