Delegation of permissions to join computers to domain

Hi

Am having some issues with delegating permissions to users for joining machines to the domain.

I have delegated permissions to a group of users which allows then to join machines to the domain, they can join and disjoin but the only problem is they cannot rejoin if the computer account still exists. 

They get the following error

The Join operation was not successful, This could be becuase an existing computer account having name xxxxxx  was previously created
using a different set of credentials.

Access Denied

Can someone tell me what extra delegation permissions i need to give to these users to be able to do this.

Thanks