AltsViewer is a simple open-sourced plugin that allows you to see players alts on your server.

For the moment only OP players or console can execute commands

/alts <player> *Shows player info*
/alts help *Shows all the commands in-game or in console*
/alts view <player> *Shows if the specified player have alts*
/alts ipview <ip> *Shows players that played on that ip*
/alts reload *Reload the config of the plugin*

Both commands to check players alts works even if the player is offline.

Features:
If a player that has alts joins, it will warn all OPs (can be disabled on config):

[✅] Plugin optimization
[✅] See if a player have alts even if is offline
[❌] Add more options in config
[❌] BungeeCord support

Feel free to suggest updates ^^.