Oh dear... this sounds like another Mt. Gox waiting to happen. In what world will this not be exploited sooner or later?
I'm (pretty) sure this is a result of them using MongoDB. A friend of mine also accomplished a double sell (BTC -> USD, -1x credit on the BTC but +2x on the USD), he ended up keeping it and never hearing from Coinbase.
Long ago I tried to withdraw my BTC, but after clicking the withdrawal button the browser tab was stuck waiting for the HTTP response. So after a while I opened a second tab and tried to withdraw again. Both withdrawals went through and I ended up with a negative BTC balance on Coinbase. I sent one of the transactions back so that my balance ends up at zero again.