crash when running mv clone command inside command block

[stdio2016]

looks like race condition
step to reproduce:

1. place a command block
2. type command mv clone world w2
3. place a button next to command block
4. click button

Description: Exception in server tick loop

java.util.ConcurrentModificationException
at java.util.LinkedHashMap$LinkedHashIterator.nextNode(Unknown Source)
at java.util.LinkedHashMap$LinkedValueIterator.next(Unknown Source)
at net.minecraft.server.v1_15_R1.MinecraftServer.b(MinecraftServer.java:1064)
at net.minecraft.server.v1_15_R1.DedicatedServer.b(DedicatedServer.java:399)
at net.minecraft.server.v1_15_R1.MinecraftServer.a(MinecraftServer.java:984)
at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:824)
at java.lang.Thread.run(Unknown Source)

Running: CraftBukkit version git-Spigot-800b93f-8160e29 (MC: 1.15.2) (Implementing API version 1.15.2-R0.1-SNAPSHOT) false
Plugins: { Multiverse-Core v4.1.0-b775 com.onarandombox.MultiverseCore.MultiverseCore [Rigby, fernferret, lithium3141, main--, dumptruckman],}

[benwoo1110]

Confirmed this error occurring when a world is loaded/unloaded with command blocks, but it looks more like a spigot bug.

[benwoo1110]

Duplicate of #2560