Yep, they totally can not charge 2 tenants rent at the same time!
We just went through the same thing!
It may also be worth checking if they have increased the rent on the property as well, as I know in SA then if they have done this, the difference between your rent and the new rent is to come off of the break lease costs as it's seen as the landlord making a profit from you breaking the lease.
For example:
If you had 2 months to go on your lease and your rent was $100. If the landlord increased the rent to the new tenants for $110, he'd be making a profit of $10 a week.
Therefore 8 weeks x $10 is $80, which would then come off of your break lease costs. We got out of having to pay any break lease costs as the landlord had put the rent up quite substantially. We had to fight for it and almost ended up at the tribunal, but it's well worth it!
Hope this helps!