An attached home means that a home shares a common party wall usually on both sides of the property. A detached home means that a home is free of any shared walls and stands alone.
Duplexes are described as semi-detached but i have also seen it described as semi-attached. In either case they mean the same thing depending how you refer to how they are attached. If you are referring to the point where the two units are attached then they are “semi-attached” at this point. If you referring to the fact that the overall duplex house is detached from any other property then you are referring to the fact that it is “semi-detached”.
In fact the prefix “semi” means half, so in essence when you say semi-detached or semi-attached you are basically saying the home is half-detached at one side or half-attached at one side.