Sorry, I haven't tested it thoroughly, it was more like a hint how you could solve the issue. What I meant was:
Find users with this email.
if([there is such user] &&
[the user we found is not the one we are upgrading]
This would mean that when we find at least one user with the email of the user we are updating, we should first check if it is not the same user. If it's not, an then exception is thrown. Also, it is wrong to have more than one such users.
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