Can I do an idempotent emit in my projection?

I have many visits-sometenant-userid streams, in which a user can have many visits to a tenant.
I would like to emit to a sometenant-users stream for each unique user to a tenant. That is a stream for each tenant with its unique users (no dups).

I tried passing my user identifier as the eventId in the emit, but it just assumes that is part of the data packet. Is there a way to do this in a projection?