Protect against duplicate logins

[AlexMorson]

1. Create two logins with the same address and username, both using the system credential store.
2. Try to delete both logins.

When trying to delete the second login, we get could not delete password from system credential store: No matching entry found in secure storage and the login is not deleted.

[Humandoodlebug]

Ah, I guess the second login would overwrite the credentials of the first in the credential store. We should probably check for existing credentials when creating a new user.