Possible Race Condition with RCON

I have a script that runs when people log in. It attempts to clear player inventory of contraband blocks that users might somehow obtain.

Usually, it runs without error, but I'm seeing intermittent errors in the console like the following, that are probably due to a race condition:

[19:57:34] [RCON Client #744/ERROR]: Couldn't process command: 'clear @a[m=0] minecraft:sponge'
java.util.ConcurrentModificationException
        at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819) ~[?:1.7.0_07]
        at java.util.ArrayList$Itr.next(ArrayList.java:791) ~[?:1.7.0_07]
        at aht.a(SourceFile:2208) ~[minecraft_server.jar:?]
        at ab.b(SourceFile:283) ~[minecraft_server.jar:?]
        at x.a(SourceFile:46) [minecraft_server.jar:?]
        at net.minecraft.server.MinecraftServer.g(SourceFile:709) [minecraft_server.jar:?]
        at ov.run(SourceFile:80) [minecraft_server.jar:?]
        at java.lang.Thread.run(Thread.java:722) [?:1.7.0_07]