Manipulate Pandas Index column on write
See here for the original answer.
According to this thread, the problem can normally be avoided by skipping calling reset_index.
If that doesn't work for you, one alternative solution that was offered is to try the following:
df.to_csv(" file_name.csv ")
df_new = pd.read_csv("file_name.csv").drop(["unnamed 0"], axis=1)