duplex or a house... ?? owner builder ??

My parents just recently brought a property and planing to build next year but we don't know whether to build a duplex or house ? can anyone give me some advice on it... whats the advantage and con of duplex.. thanks
i got a younger brother the plan was to build a duplex one for me and one for my younger brother in the future
and just wondering if i should do owner builder or just hand the whole project over to a builder ?
i got a little idea how the steps goes n been doing some research on building.... been going past a few sites been built near my area and have a peek every now and then...

Soor Mr Chook- councils / areas / builders and investors will all have a varying opinon on this.

I just had to post cos of the name.

mr chook - Priceless