def _validate_names(names):
    """
    Check if the `names` parameter contains duplicates.

    If duplicates are found, we issue a warning before returning.

    Parameters
    ----------
    names : array-like or None
        An array containing a list of the names used for the output DataFrame.

    Returns
    -------
    names : array-like or None
        The original `names` parameter.
    """

    if names is not None:
        if len(names) != len(set(names)):
            msg = ("Duplicate names specified. This "
                   "will raise an error in the future.")
            warnings.warn(msg, UserWarning, stacklevel=3)

    return names