CSV parsing fails when duplicate columns names are present

[texodus]

Bug Report

Steps to Reproduce:

Load a CSV with duplicate column names and different types. The best way to trigger this segfault is to force the parser to try to read an e.g. int column as a string:

A,A
test,1

Repro at #1910

Actual Result:

Segfault

Environment:

Browser

Additional Context:

Better error reporting is a must, but automated column de-deduping would be preferable.