BUG: DataFrame.setitem not raising ValueError when rhs is df and has wrong number of columns by phofl · Pull Request #39341 · pandas-dev/pandas (original) (raw)

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let me rephrase this:
We don't get there, becasue 3227 would raise. If we would change the condition to recognize duplicates, we would get a problem in 3229, because the lenght of both lists would not match.
If we also fix this, we have to check that in case of a key which is duplicated in the columns, we select two columns and not only one in 3230 on the rhs

Edit: Similar problem if rhs would have duplicates.