At the moment, there is no warning when a user who wants to create a new account while logged-in.
There is only an optional field where the use can specify why they need a new account. But is is not really a way to warn them about the fact that they are already connected to their account.
A person is directed to create an account on Wikipedia (as part of a campaign or editing event) which provides a link to the "Special:CreateAccount". The person has previously signed up and remained logged in, but has forgotten, so they select the page. They do not realise they are creating an additional account/
Show a warning dialog when a logged in user accesses the Special:CreateAccount page via a call-to-action