Pandas concat failing

I believe that this error occurs if the following two conditions are met:

1. The data frames have different columns. (i.e. (df1.columns == df2.columns) is False
2. The columns has a repeated value.

Basically if you concat dataframes with columns [A,B,C] and [B,C,D] it can work out to make one series for each distinct column name. So if I try to join a third dataframe [B,B,C] it does not know which column to append and ends up with fewer distinct columns than it thinks it needs.

If your dataframes are such that df1.columns == df2.columns then it will work anyway. So you can join [B,B,C] to [B,B,C], but not to [C,B,B], as if the columns are identical it probably just uses the integer indexes or something.