Duplicated group names break incremental sync

In full sync mode, groups with duplicated names are dropped. However they're not dropped in incremental sync. This may cause the incremental sync to fail.

Steps leading to an error:

1. Duplicated groups in AD are created.
2. Directory in Crowd is created.
3. Initially it'll do full sync, but the duplicated names won't be synced.
4. Incremental sync will see that the GUIDs of those duplicated groups are missing.
5. It'll ask for data since the last full sync. Duplicated groups were created before last sync, so they won't be returned.
6. Incremental sync will fail, because it doesn't see the missing GUIDs in the delta results.