Duplex houses are typically not completely soundproof, especially when it comes to airborne sounds like singing. While the construction of the house and the materials used can affect the level of sound transmission, it's difficult to completely eliminate sound transfer between the two halves of a duplex.
Soundproofing measures can be implemented during the construction phase to minimize sound transmission, such as using sound-absorbing materials, insulating walls, and sealing any gaps or openings. However, some level of sound can still travel through common walls, floors, and ceilings.
If you sing loudly in one half of a duplex, it's likely that the sound will be audible to some extent in the other half. The degree of audibility will depend on various factors, including the thickness and construction of the walls, the distance between the units, and the specific soundproofing measures taken.
To minimize sound transfer between duplex units, additional measures can be taken, such as using double-layered drywall with sound-dampening properties, adding insulation in the walls, and installing acoustic seals around doors and windows. Consulting with a professional contractor or sound engineer can provide specific recommendations tailored to your situation and help achieve better sound isolation between the units.