Hi @afonso,
There are 4 built-in properties in Users collection: Email, Password, Username and Full Name. They can’t be deleted by design.
The only property which is checked for uniqueness is Email, meaning that if one tries to create a user with the same value in Email field, this will fail. Just in case, adding a space before/after isn’t checked, which creates another potential headache for support 
However (and unfortunately in my point of view) these properties could be (a) renamed and (b) rearranged together with other custom-created properties. I’ve seen that quite a lot of times.
The issue is, that if app Maker renames “Email” property to something else (e.g. phone), and renames other “built-in” properties, it will be quite difficult to find which exact property is that “built-in email” which is checked for uniqueness. Especially if Maker decided to create another property and name it “Email”.
The only hint is that built-in properties couldn’t be deleted (you see the lock on the right side). At the same time any first property in the collection couldn’t be deleted, which adds some intrigue when you try to find the original email property.
So imagine Users collection with 20+ various properties, original properties rearranged and renamed, and you are creating the form to log in / sign up a user. This is what I call a “mess” 
.
If you’re 100% sure you don’t have such case and you’ve set up your sign-up form correctly - inspecting browser error console can give a hint about the error’s reason. Sometimes Adalo DB has glitches.
Hope this helps
Best regards, Victor.