digg drops a cookie when you use them once and if you use them again with a different account on same IP as before, you must make sure to FLUSH your BROWSER's cache and reboot your DSL modem to get another IP

and you're good to go...
[edit] ah! i forgot to tell ya, if you're using private IP scheme, then you're doomed coz, you're gateway IP remains the same.
get on public IP
Cheers
Rugby