Tycho, When I initially made the double spend page there was only one transaction, now there are 6. Is that you testing it?
No, I'm not.
I think that it's some broken client or attack against some broken client.
You can try to monitor for transactions with multiple instances of same input, that's what happens too, as my monitoring nodes show.
I heard that there was some old client storing them in memorypool or doing something else.
UPD: Also it may be caused by people restoring their wallets from backups or using wallet.dat copy accidentally.
What we should especially care for is double-spend attempts in orphaned branches.