The reason a duplicate account is created is that a profile for the user can be created through the Inquiry Form and the Application. As an administrator, the Create Inquiry task in onBoard can create a new candidate record. If this is done first, and no Username is added to the profile, the parent can elect to create a new user from the apply link and it will create a new (duplicate) user.

Now if a Username was added by the administrator in the Create Inquiry task, and the parent tries to create a username in the Application that is the same as the one that exists in the Inquiry, the system will flag it and the duplicate profile will not be created. No two or more user accounts can share the same Username. This can help to avoid duplicate users being created in the system. 

Also, there is a task in the application settings to turn off profiles from being created through the application process. This can be set by going to:
  1. Select the onBoard persona
  2. Hover over the settings icon, and select Application Forms
  3. Click Login Setup
From here, use the Allow new applicants to create their own account to toggle on/off the ability to create accounts from the application form. 
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