Email is already running only if the account is verified

Question

I would like to redefine the action that says "Email is already completed" in Devise

I am creating an application in which a user can invite someone to several elements (todo, events, folder, etc ...) through his email. To avoid multiplication of invitation tables and to increase scalability. My idea was to create a user with only an email. This allows all users of the application to contact a unique user (via his email), even if he is not registered. Instead of multiplying invitation elements. However, the basic “Design” configuration does not allow this organization. I would like to repeat this registration step.

This is what I want to do:

if this mail was already been taken
  if the account related has already confirmed his email
    notice = 'Email is already taken'
  else 
    send a confirmation on the mail
  end 
else
  Create User
end

I have already consulted the Devise_invitable gem , but this is not quite what I want to do.

Has anyone ever tried to do this? If so, how?