The InspIRCd 1.2+ spanningtree protocol adopts a TS6-like nickname collision algorithm which minimises the impact of nickname collisions and prevents most if not all KILLs when a collision does occur.
The rules for nickname collision are as follows:
If the user@ip of the two colliding users are equal: Force nick change on OLDER client which changed their nickname first (this is to prevent penalizing users who have reconnected, so that their 'ghost' does not win the collision).
If the user@ip of the two colliding users differ: Force nick change on NEWER client which changed their nickname last (this is to help prevent users camping on nicknames, causing a collision and stealing that nickname during the resync).
If the timestamps of the clients are equal: Force a nick change on both.
Where nickname changes are enforced by this algorithm, the nickname is changed to the users UUID. Because the user's UUID contains a leading digit, it is impossible for any user to NICK to their UUID or anyone elses UUID and 'camp' on it. Only a server may force a nick change to such an 'illegal' nickname.
IMPORTANT NOTE: In a vanilla install of InspIRCd, there is no KILL message here for any collision scenario. However, it is possible that a third party module may deny the nickname change to UID, which will cause a last-resort Nickname collision KILL. Understandably, this is not a desirable situation, so care should be taken not to run such modules if this bothers you.