Kicked: Duplicate IP. -is this the norm?

(Kicked: Duplicate IP. Contact the Administrator.)

I have not met or thought before. I did not install anything controlling the connection to the server. The player wrote that his friend from the same IP can not connect. Can anyone know this?

Pretty sure that is not normal, so would have to be from a plugin or tool.
thank! Found several error records. disabled mods, but only tomorrow they will check (the situation is not standard - they play with 1 IP).
At the beginning, these records were ignored - I thought that "Recycle" could not be connected with a connection error..

(21:20:45) | Failed to call hook 'OnPlayerDisconnected' on plugin 'Recycle v2.1.9' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.Recycle.OnPlayerDisconnected (BasePlayer player) [0x0000c] in <5838072522f04212b3ff4af23d6860c9>:0 
  at Oxide.Plugins.Recycle.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x0045e] in <5838072522f04212b3ff4af23d6860c9>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <9affce1cd15c4ec183941adef8db1722>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <4452f821def6406d834e4149849fe7ea>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <4452f821def6406d834e4149849fe7ea>:0
(21:20:45) | [Welcomer] ✘ Eygeny покинул сервер (Kicked: Duplicate IP. Обратитесь к Администратору.)​

but I'm not sure yet, I don't know if that is the reason
It isn’t coming from Recycler.
Ok, I do not understand the code. I just got rid of the console from all sorts of spam generated by the hosting mods (the simple solution is to remove them and put the normal uMod)
This is the only problem that has appeared in the last week, so just ignore it for now.

but information about such an error (Kicked: Duplicate IP. Contact the Administrator.) - I could not find it on the network.
Do you have any RCON tools connected?
Sounds and looks to be from RustAdmin.
thank! Can you recommend an alternative?)
It's likely an option in RustAdmin that you can disable.
Thanks again and sorry to bother you!