Reindexing only valid with uniquely valued Index objects doesn't make sense

Question

Nothing I've read about this error has made it any clearer. I have two dataframes with different header names even though they are the same. I want to append one into the other but I keep getting:

pandas.errors.InvalidIndexError: Reindexing only valid with uniquely valued Index objects

From what I've been able to gather this means that there are multiple indicies values that are the same within each individual dataframe. One answer recommended to do this: df3 = df3.loc[~df3.index.duplicated(keep='first')] for each dataframe but it doesn't make any difference.

Here is the code and heads of each df. I would really appreciate anyone being able to help me understand this:

    df2company_col = get_company_name(df2)
    df2website_col = get_website(df2)
    df2postal_col = get_five_postal(df2)
    df2phone_col = get_phone_column(df2)
    df2state_col = get_state_column(df2)
    df2 = df2[[df2company_col, df2website_col, df2postal_col, df2phone_col, df2state_col]]
    df2 = df2.loc[~df2.index.duplicated(keep='first')].reset_index(drop=True)
    print(df2.index)
    print(df2.head())

    df3company_col = get_company_name(df3)
    df3website_col = get_website(df3)
    df3postal_col = get_five_postal(df3)
    df3phone_col = get_phone_column(df3)
    df3state_col = get_state_column(df3)
    dct = {df3company_col: df2company_col,df3website_col:df2website_col ,df3postal_col: df2postal_col, df3phone_col: df2phone_col, df3state_col: df2state_col}
    df3 = df3[[df3company_col, df3website_col, df3postal_col, df3phone_col, df3state_col]]
    df3.rename(columns=dct,inplace=True)
    df3 = df3.loc[~df3.index.duplicated(keep='first')].reset_index(drop=True)
    print(df3.index)
    print(df3.head())

    # When I uncomment the two lines below is when the error throws
    # df_leads_accounts = pd.concat([df2, df3], axis=0)
    # print(df_leads_accounts)

Here are the outputs of the print statements:

RangeIndex(start=0, stop=1128291, step=1)
                       Company / Account                       Website Zip/Postal Code       Phone  State/Province
0             KAREN GREEN CONSULTING LLC                           NaN             NaN  9046140587         Florida
1                   MEMBERS CREDIT UNION                           NaN           27103  3367749997  North Carolina
2  FYZICAL THERAPY  BALANCE CENTER TYLER  http://www.fyzical.com/tyler           75703  9033639252           Texas
3                   GLASFLOSS INDUSTRIES                 glasfloss.com           75226         NaN           Texas
4                            PAWN 2 CASH        https://pawn2cash.org/           47714  8124773781         Indiana
RangeIndex(start=0, stop=622914, step=1)
                                 Company / Account                             Website State/Province       Phone State/Province
0  GREENIX PEST CONTROL SOUTH 800 EAST OREM UT USA                                 NaN            NaN  1385219070            NaN
1                                   SMASH MY TRASH                www.smashmytrash.com          16464  1636368749          16464
2                              WALLBEDS N MORE INC               www.wallbedsnmore.com            NaN  6198861828            NaN
3                                  R  S MECHANICAL                  www.comfortguy.com            NaN  9193028956            NaN
4                    FINAL FRONTIER RESCUE PROJECT  www.finalfrontierrescueproject.org            NaN  1512817944            NaN