LockSet: allow lists with duplicate values

If a list with a duplicate value is passed to a lockset what the code
now does is to try to acquire the lock twice, generating a
double-acquire exception in the SharedLock code. This is definitely an
issue. In order to solve it we can either forbit double values in a list
or just delete the duplicates. In this patch we go for the latter
solution, removing any duplicate values when creating the acquire_list.

Reviewed-by: imsnah