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...