Upon double clicking (or any amount more, really) the server in the server listing, the client will attempt to login multiple times to the target server, causing pandamoneum and breaking joins on vanilla and modded (bukkit/spigot/bungee) servers. Note however this issue does not exist if you specifically click the "Join Server" button at the bottom of the listing.

A few client logs showing the issue:

[18:21:22] [Client thread/INFO]: Setting user: Rogue
[18:21:22] [Client thread/INFO]: (Session ID is token:BLAHBLAHBLAH
[18:21:24] [Client thread/INFO]: Reloading ResourceManager: Default, faithful32pack.zip, NoPortalSound.zip
[18:21:25] [Sound Library Loader/INFO]: Starting up SoundSystem...
[18:21:25] [Thread-5/INFO]: Initializing LWJGL OpenAL
[18:21:25] [Thread-5/INFO]: (The LWJGL binding of OpenAL. For more information, see http://www.lwjgl.org)
[18:21:25] [Thread-5/INFO]: OpenAL initialized.
[18:21:25] [Sound Library Loader/INFO]: Sound engine started
[18:21:27] [Client thread/INFO]: Created: 1024x1024 textures-atlas
[18:21:32] [Client thread/INFO]: Connecting to mc.hypixel.net, 25565
[18:21:33] [Client thread/INFO]: Connecting to mc.hypixel.net, 25565
[18:21:34] [Client thread/INFO]: Connecting to mc.hypixel.net, 25565
[18:21:34] [Client thread/INFO]: [CHAT] [STAFF] [ADMIN] Rogue joined.
[18:21:34] [Client thread/INFO]: [CHAT] Rogue joined.
[18:21:34] [Client thread/INFO]: [CHAT]
[18:21:34] [Client thread/INFO]: [CHAT] [LibsDisguises] There is a update ready to be downloaded! You are using v1.0, the new version is v8.6.7!
[18:21:34] [Client thread/INFO]: [CHAT]
[18:21:34] [Client thread/INFO]: [CHAT] NCP: Your configuration might be outdated.
[18:21:34] [Client thread/INFO]: [CHAT] Some settings could have changed, you should regenerate it!
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 54, 727
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 724
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 723
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 726
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 725
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 727
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 54, 723
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 54, 725
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2523, 22, 743
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 54, 761
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 54, 763
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 54, 765
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 762
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 761
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 764
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 763
[18:21:35] [Client thread/INFO]: [CHAT] Unable to locate sign at -2528, 53, 765
[18:21:35] [Client thread/INFO]: [CHAT] A disconnect occurred in your connection, so you have been routed to limbo!

Note the multiple "Connecting to..." lines, these are separate attempts to simultaneously log into a server, breaking it.

Here's an example of a single login attempt from a server's perspective: http://i.imgur.com/dBpqE5O.png

Here's a relevant race condition/NPE to the problem I managed to find spammed after a failed attempt:

[18:20:08] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.8.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.8.0_51]
at g.a(SourceFile:45) [1.8.9.jar:?]
at ave.av(SourceFile:881) [1.8.9.jar:?]
at ave.a(SourceFile:325) [1.8.9.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.9.jar:?]
Caused by: java.lang.NullPointerException
at bdb.a(SourceFile:205) ~[1.8.9.jar:?]
at gv.a(SourceFile:153) ~[1.8.9.jar:?]
at bcy.a(SourceFile:355) ~[1.8.9.jar:?]
at gv.a(SourceFile:144) ~[1.8.9.jar:?]
at gv$a.a(SourceFile:61) ~[1.8.9.jar:?]
at fh$1.run(SourceFile:13) ~[1.8.9.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_51]
at g.a(SourceFile:44) ~[1.8.9.jar:?]
... 3 more