Steps to repro:
1) Create a new test
2) Go into Local Data screen
3) Rename the first column to "abc" and save
4) Rename the first column to "ABC" 
Expected: column is renamed no problem 
Actually: You get an error: "Column name already exists 'ABC' ". 

So it would seem that the column also checks itself when checking for duplicates. And the check is not case sensitive. This is obviously a minor bug. 

Workaround: name the file something else and then rename it back to the desired named