Does house insurance for a duplex cost more than for single family?

Yes because part of the property is not owner occupied. Regular non-owner occupied is always alot more, but a duplex may not be as drastically different.

I own both single family and duplexes that I rent out. The cost is not higher per address, but the insuance is calculated as two seperate residences for the duplexes. I do not pay them as a single building.