[DF] Avoid duplicates in list of columns returned by GetColumnNames
[DF] Avoid duplicates in list of columns returned by GetColumnNames

The same column name can appear as a Redefine'd column and as the
original column in the data source.

This fixes the "repeated column names" part of #9240.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root