We use a little bit different approach for user logon - they provide email address (instead of username) and password (to simplify: email address is username).
We have also Ldap membership provider enabled and we allow ldap
users to login on the front-end. Everything works fine, but admin users
can create accounts in the backend (Sitefinity admin), and we need to
restrict email addresses to be unique (in other words: admin cannot
create a new user with email address that exists for Ldap user). We implemented this on the front-end by checking with LdapMembershipProvider if provided email address exists, if not then we call CreateUser on UserManager, but we cannot add any custom validation in Sitefinity backend to create user form.
Is there any possibility to achieve this ?