title: Pandas merge: Cannot use name of an existing column for indicator column
body: <p>I have two dataframes:</p>
  <pre><code>df
  side    amount  price   fee datetime    symbol  account_id  order_id    trade_id    exchange
  4511    buy 4397.426832 6.84    4.397427    2020-08-18 13:08:15 xrpmxn  prod_xrpmxn_bitso   TX0s5HpiHH0C83xZ    17731587    bitso
  4512    buy 1.397976    6.84    0.001398    2020-08-18 13:05:02 xrpmxn  prod_xrpmxn_bitso   TX0s5HpiHH0C83xZ    17731587    bitso
  4513    buy 601.175192  6.84    0.601175    2020-08-18 13:04:00 xrpmxn  prod_xrpmxn_bitso   TX0s5HpiHH0C83xZ    17731587    bitso
  4514    buy 10000.000000    6.86    10.000000   2020-08-18 11:34:35 xrpmxn  prod_xrpmxn_bitso   TX0s5HpiHH0C83xZ    17731587    bitso
  4515    buy 300.000000  6.87    0.300000    2020-08-18 09:31:00 xrpmxn  prod_xrpmxn_bitso   TX0s5HpiHH0C83xZ    17731587    bitso
  
  df_prod
  
  ts  exchange    symbol  trade_id    order_id    side    price   amount  account_id  fee
  4308    2020-09-16 16:20:50.307 bitso   xrpmxn  18011784    Gtr8LC61Vdv9e1tP    buy 5.1639  49.000000   prod_xrpmxn_bitso   49.000000
  4309    2020-09-16 16:21:54.502 bitso   xrpmxn  18011803    Gtr8LC61Vdv9e1tP    buy 5.1639  52.828604   prod_xrpmxn_bitso   0.052829
  4310    2020-09-16 16:23:05.198 bitso   xrpmxn  18011853    Gtr8LC61Vdv9e1tP    buy 5.1639  29898.171396    prod_xrpmxn_bitso   29.898171
  4311    2020-09-16 16:25:46.094 bitso   xrpmxn  18011893    cHjL6T0emme2yO8K    buy 5.1559  14347.930935    prod_xrpmxn_bitso   14.347931
  4312    2020-09-16 16:27:22.724 bitso   xrpmxn  18011914    jbrHspMDN6mSSoTl    buy 5.1544  138.738364  prod_xrpmxn_bitso   0.138738
  </code></pre>
  <p>I am trying to compare if each row in df_prod exists in df or not, I tried the following:</p>
  <pre><code>import numpy as np
  df = pd.merge(df, df_prod, on=['amount','price'], how='left', indicator='Exist')
  
  df['Exist'] = np.where(df.Exist == 'both', True, False)
  print (df)
  </code></pre>
  <p>But then I have the title error, and I do not understand it, any idea why? Thanks!</p>