Is this a bug? Not sure, but you should be aware, because I think there’s something you guys could do about this.
I just had a user log in to the app on two different devices. On one device, he typed his email with all lowercase and on the second device (likely because of autocapitalization), he typed it with the first letter capitalized. The google spreadsheet function =UNIQUE really is just that—totally unique. Including case sensitivity. So in the user sheet which uses =UNIQUE to duplicate all log-ins from the App:Login sheet, that second login was recognized as a newly added user.
I’ve fixed this by creating a second column that duplicates B2 in App: Login called Email Lower with the following function =ARRAYFORMULA(IF (B2:B<>"", LOWER(B2:B),"")) which creates a new column of the same B column data where all emails are lowercase. Then in the user sheet, I have =UNIQUE pointing to the D column not the B column.
But this could be easily solved by just forcing that text field for signing in with your email address to only type lowercase. Possible? Would alleviate that extra column I just put in.
In the interim, I suggest other gliders who rely on that kind of thing to look at this.