def append(df, entry):
    """
    This function adds a new entry in the DataFrame 
    if the entry timestamp does not exists 

    Parameters
    ----------
    df: pandas.DataFrame
        Fataframe to record
    entry: pandas.DataFrame
        entry dataframe
    
    Returns
    -------
    A pandas with the updated DataFrame
    """
    if entry.index[0] not in df.index:
        return df.append(entry, ignore_index=False)
    else:
        return df