Alt Checker
Alt Checker is a minecraft plugin that allows you to see your player's IP address, as well as all the accounts that have logged into your server with the same IP address.
The plugin stores IP addresses and ip address lists in the config, so do not mess with it. Upon logging in, a player will have their IP stored and if their IP does not already exist in the config, it will create a directory. If it does exist, it will add the player to an already existing list directory. Due to the nature of YAML files, the plugin will log IPs in an odd way. The way they are logged does not matter though, so the plugin will work fine even if IPs stored in the list in the config are done oddly.
Commands:
/alts <player>: Displays the players IP adress and then will display all the accounts that have logged into the server with the same IP.
Permissions:
altcheck.check: Allows a player to use /alts command. Given to OPs by default.
altcheck.bypass: Will not track the IP of a player with this permission node.
The plugin will work on offline players, so long as they have logged onto your server before while the plugin is installed.
Example:

Versions:
The current version is working on test servers up to 1.11.2.
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Posted Aug 3, 2016@GamingChair
When will this happen?
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Posted Jan 10, 2017I ended up taking a considerably lengthy break from anything minecraft related, but as far as I can tell, the plugin still works fine with other minecraft versions, including 1.11 (which I tested).
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Posted Mar 29, 2016Is this UUID Compatible?
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Posted Mar 11, 2016@ThisUsernameIsMine
If you are referring to the way it is logging the IP in the IPList, it logs that way because I get the player's IP address and then store it as a string. YAML files register .'s in strings as creating a new subcategory, resulting in something like that. I haven't found any particularly easy way to fix that. Does the plugin still display the players on the same IP upon using the command correctly? If not, I can try updating it to a 1.9 version. (I will update to 1.9 eventually anyways.)
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Posted Mar 10, 2016Does this work under 1.9 (Spigot)?
It logs something but what it logs isn't correct (atleast, not in my case):