I have an unconfirmed payment from 11/7 and a negative balance with no double payment visible.

On the Slush web site
https://mining.bitcoin.cz/accounts/profile/ it says
" Quick updates
08.11.2013
Some recent payouts didn't go thru bitcoin network because of unsatisfied transaction fees. We're finding a soluton and all payouts will be processed soon"
Hopefully they will fix this soon, or try to contact Slush support email or the Live Support.
In my case the double payments were not consecutive. looking at my wallet transactions, stats (I copy and keep my Slush stats) and the payout history I managed to figure it out more or less...
The account message changed
from: "Notice: The last payment to you was sent twice by a mistake. This has caused that your confirmed reward is temporarily below 0 BTC. We're sorry for the inconvenience."
to: "Notice: Some payments from 2013-11-08 were sent twice by a mistake. This has caused that your confirmed reward is temporarily below 0 BTC. We're sorry for the inconvenience."
The transaction from 11/7 confirmed at last. The double payment showed up about an hour ago.
The account message was incorrect and then premature in regard to my account.
Looks like things are making sense now but in any case it is a little bothersome.