A duplex is typically one structure with two dwellings in it. Typically they share a wall. So while most homes are  stand alone and have walls leading to outside on all four sides, a duplex would have one of those walls lead directly into the other unit. No outside in-between the two houses. 

It's very efficient from a construction and utilities perspective. Heating and cooling tends to be cheaper for both parties. And construction costs are less, since you have essentially one less exterior wall per dwelling. So there are some advantages to the duplex.

However, I am a duplex hater. From this soapbox I will explain why. 

A duplex suboptimal for both "desirable" housing density situations. If you want your property to be your property a duplex fails. Your neighbor starts a kitchen fire? That's your problem. You want privacy? Your neighbor can hear every word you say. You want to be left alone and not deal with other people? Too bad there's literally another family living in your building. To have all those things you need a single family home. Which at least in America is considered the most desirable living situation. 

But what about the advantages of duplexes I was talking about? Well it turns out those are also sub-optimal compared to say condos or townhomes. Living in a high density area has advantages. Sharing walls has advantages. Heating and cooling is cheaper. Utilities construction and maintenance is cheaper. Things like public transportation are cheaper. In a sense all the community features are cheaper with higher density living. The sense of community can also be a great thing. 

But with a duplex? Well that's just one stand alone house cut down the middle. It's still on a plot of land, so it has all the disadvantages of single family homes and all the disadvantages of sharing a wall. Yes it's slightly more population dense than all single family homes. But typically not enough to be worth it to run say a subway line or whatever. 

Duplexes are a terrible, terrible idea. The only benefit is that the guy who builds it gets to sell two houses for the cost of building one. It does not stand up to the livability test at all. (Except in rare situations where you actually are friends/family with whoever else lives in your building.)