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Probably I have added a column named 'username' to the Users table in database eventhough this already existed.
Because of this I can't remove this column or add another one. If I change the value in one username column the other is updated to.
Can you remove the 'username' column for me? 
That's probably the reason, we'll add validation to column with such name can't be added to User collection. Probably the simplest way to solve this right now is do delete this database and create a new one.
Creating a new db shouldn't be a problem. But in new db I still can't add new column to the Users table.
I have had 3 different scenario's:
Create column button doesn't do anything when I click it.
Create column popup is closed but no new column. Can do same thing multiple times without problem.
Create column popup is closed but no new column. If I try again I get message that column is already existing, but not visible in list.
I am using Chrome btw.
Create new column doesn't work in Firefox.. but I was able create new columns in Chrome.
How did you create the new 'username' column (from screen shot)?
I will try different browser and have a look.
About 'username' column: I have no idea when/if I have done this. I was not using the table in code, and when I wanted to use the table I wanted to add name field. At that moment I saw the duplicate columns.
This will be fixed in production on Thursday.