The _checkColumns function silently ignores columns with repeat names

If two or more columns in a data set have the same name, the program will record the index of the first one it encounters and ignore the rest, meaning the data in the other columns is thrown out. While generally not a problem, it may be better to alert the user to the duplication.

Locate the _checkColumns function and trace how it handles columns with duplicate names. Confirm the current first-match behavior, then determine how duplicate names should alert the user without silently discarding the other columns; done means the duplicate case is surfaced clearly and existing unique-column behavior remains intact.

Written by the indexing model from the issue text.