Steps to reproduce:

1. Create a native local domain with no maxID

2. Create a user without specifying the userid

   sss_useradd user1000

3. Note {{{user1000}}}'s UID (e.g. 1000).

4. Verify that the following command indicates that the nextId = 1001

   ldbsearch -H /var/lib/sss/db/sssd.ldb 'nextid=*'

5. Create a user with the nextId specified explicitly

   sss_useradd -u 1001 user1001

6. Verify that {{{user1001}}}'s UID is 1001

7. Create another user without specifying the ID

   sss_useradd user1002

Expected behavior:
user1002 has UID 1002 and nextID is set to 1003

Actual behavior
user1002 is given UID 1001 (already in use by user1001) and nextID is set to 1002

While the use of the -u option is discouraged, it is still supported. We should ensure that our autonumbering system takes these users into account.