Oh, I could click my red ruby slippers three times but I'm not in Staten Island anymore. I do miss the Rock, though. Growing up on Staten Island, New York the duplex was a way of life. I would visit friends who lived on one side of the house, while their grandparents or uncle and aunt lived on the left side.
Monmouth County tends to shock transplants like myself. Where are all the duplex homes? We moved 15 years ago to Colts Neck from the Westerleigh and Willowbrook neighborhoods. My Missus is a Travis native.
Back to the duplex. What is it? Like it sounds, a duplex is two vertical residences built side-by-side to mirror each other.
What of the Monmouth County duplex? Well, it is a rare building breed. The REALTOR industry sold over 5,000 single family homes last year. The overwhelming majority of these sales were detached houses.
Even the term duplex is loosely used in Monmouth County. The more popular description is attached housing. Here are the fifty (50) closing sales of attached single family homes in Monmouth County last year.
There are fifty (50) towns in Monmouth County, so they're all spread out. So fellow Staten Islanders one element of the Rock you will miss is the duplex. Unless you build it yourself. I can help you find raw (vacant) land.

I am sensitive to recent buyers of property, so I blocked out the address in my data table. Although these sales are public record and printed in the local papers, I realize folks are protective of their investments.
If you would like additional MLS data or real estate information (or a full version of my analysis) please contact my office.
Me? I'm a kindler, gentler REALTOR ;)
Thinking of buying or selling? Still a good time either way. Monmouth County has proven very resilient to the global economic forces punishing real estate values in other parts of the country.
We represent buyers and sellers of condos, single family homes and investment properties through most of Middlesex, Monmouth County real estate and Ocean County real estate. 
You can call Andrew at (732) 431-9003 or email me or visit me at my company website.
Andrew J. Lenza, ABR GRI MBA Broker/Owner
ANDREW J. LENZA REALTY
73 State Highway 34 Colts Neck, NJ 07722
Office Telephone (732) 431-9003
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Duplex is a term that is rarely understood in my area. It is a zoning category with 2 deeded properties, attached, each sold separtely. Townhome style, but always 2 to the lot.
I get calls from folks from NY and NJ regularly who want to buy a duplex, but they mean both units. They don't understand that a duplex is not a multiple unit building where the owner can live in one and rent out the other.
Duplex here means one unit, one deed, one half of the structure.