A "qualifying person" for Head of Household still must qualify as a dependent - if they are filing their own tax return (i.e. claiming themselves), then they will not be a "qualifying person" for Head of Household purposes.
Your return will get rejected because that person's SSN has already been used to file their "own" return.
The conditions for a "qualifying person" are stricter than those for claiming a dependent; for example, you might be able to claim a roommate as your dependent, but never as a qualifying person for Head of Household status.
In order for the TurboTax program to determine if a you qualify for Head of Household, you have to enter that person as a dependent on your return. After you answer all the questions about yourself and your dependents at the beginning of your return, we'll let you know if your situation qualifies for Head of Household.
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