Onselling before settlement…

Hi, Can someone please give me a quick outline of what happens (stamp duty wise) if I wanted to onsell a property I have a 6 month settlement on during that time…?

I'd prefer to have an "option" to buy it, but in this case it is a signed contract of sale.. Is there a way of assigning a contract of sale without stamp duty implications? Or is an option contract the only way to do this smoothly…?

Thanks,
Brett