Columns are silently lost if multiple columns have the same name

[jverkoey]

Example:

en,comment,de,en,en-GB,fr,es,it
,,,,,,,
Efficiency,,,,,,,
%@,,,,,,,

In this case, the "Efficiency" row will appear as an empty row when printed because there are two columns named en.

The fix is to not have duplicate column names, because they're being treated as keys, but this type of issue would ideally be thrown as an error to make it clear that unexpected behavior is happening.