Fresno fire says the fire started around 2 a.m. Wednesday morning in a car inside a garage of the duplex on West Avenue near Kearney.
Authorities say the fire quickly worked its way up to an attic.
Two families were living in the duplex at the time of the fire, but all 12 people made it out safely.
The carport and one apartment were damaged. Fresno fire is still looking into the cause.