fix: while creating a new account, check for his existance

[madbob]

Unsure if the duplication check should span across all existing accounts on the instance, or just those registered by the current user...

Fixes #12042

[ChristophWurst]

Thanks for the PR

[GretaD]

it works, thank you

[GretaD]

/backport to stable5.10

[GretaD]

would it be possible for you to write a quick test about this check?

[ChristophWurst]

What I'm missing

1. Same logic for occ mail:account:create or moved check
2. Strategy for existing accounts
3. (Eventually) a UNIQUE INDEX in the db because that is the only way to assure the constraint

[madbob]

1. Same logic for occ mail:account:create or moved check

I'm going to check.

2. Strategy for existing accounts

In my opinion, this cannot be retroactive. What is done, is done.

3. (Eventually) a UNIQUE INDEX in the db because that is the only way to assure the constraint

This implementation just checks that the current user do not register multiple times the same mail account, but no one specified what happens if different users register the same mail account. If allowed: it cannot be unique in the database. If not allowed: this implementation is wrong.