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14.6. Ensuring That ID Values Are Unique

It is recommended to avoid conflicting UIDs or GIDs. UIDs and GIDs should always be unique: two users should not have the same UID, and two groups should not have the same GID.
Automatic ID assignment
When a user or a group is created interactively or without a manually specified ID number, the server assigns the next available ID number from the ID range to the user account. This ensures that the UID or GID is always unique.
Manual ID assignment
When you assign an ID to a user or a group entry manually, the server does not verify that the specified UID or GID is unique; it does not warn you of a conflict if you choose a value that is already used by another entry.
As explained in Section 14.7, “Repairing Changed UID and GID Numbers”, the SSSD service does not handle entries with identical IDs. If two entries share the same ID number, a search for this ID only returns the first entry. However, if you search for other attributes or run the ipa user-find --all command, both entries are returned.
UIDs and GIDs are both selected from the same ID range. A user and a group can have the same ID; no conflict arises in this situation because the UID and the GID are set in two different attributes: uidNumber and gidNumber.
Setting the same ID for both a user and a group allows you to configure user private groups. To create a unique system group for a user in this way, set the same ID value for a user and also for a group, in which the only member is the mentioned user.