Duplicate column names are not supported

[d6y]

A CSV with duplicate column names...

Home School, Allocation, Joint School, Allocation

...modelled as: (String, Double, Option[String], Option[Double]) will fail to parse a valid row such as:

Foo, 1,,

...reporting that "Allocation" is not a number. There's some confusion over "Allocation" being required, but optional in the second usage.

Not sure what to do about this. Possibly warn if there are duplicate columns? or support duplicate columns.

@d6y needs to provide a test case to demonstrate this is actually an issue and not a figment of his imagination.