>Say you have a table xyz with a an id field named id type char.
>

SET DELETED OFF
>USE xyz EXCLUSIVE
>DELETE ALL
>INDEX ON Id TAG UniqueId UNIQUE
>
>SET ORDER TO Id
>RECALL ALL
>DELETE TAG UniqueID
>SET ORDER TO
SET DELETED ON

I made few corrections. It's important to note, that this solution requires exclusive use of the table, while Sergey's solution doesn't require it.