Duplex is defined; 2-family dwelling: a house that is divided into two halves and is inhabited by two separate families or tenants with separate entrances Encarta ® World English Dictionary © & (P) 1998-2005 Microsoft Corporation. All rights reserved.
CO description; Detached Two-Family Dwelling.
Detached Two-Family Dwelling cannot have a lot line between the units; but are "separated by not less than a 1 hour fire resistance rating". It can have plumbing, gas and electric cross the unit boundaries or party wall.
Attached Single Family Dwelling as we list them on the CO is the same as a townhouse meeting the requirements of Section R302. We do not label them as townhouses on the CO.
I just had a similar situation where there was a misunderstanding with an electrical service (two meter cans) to an Attached Single Family Dwelling, 4 units on a single lot, big mistake.
Mule; facilitated communication among all concerned parties, well done as always.