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...
Newcomers to slush pool: slush's pool IS the shit. Slush's pool is a small operation. Sometimes weird stuff happens for a while, most likely because the genius is updating his site. It ALWAYS corrects itself (at least 99% of the time). Just trust the slush.