This happened to me this morning when I was making a purchase. It threw an error on their website,saying that the charge did not go thru. So, thinking that I had entered the numbers incorrectly, I re-entered the CC numbers with the same result. I pulled out a second CC and it went thru.
Fast-forward to this afternoon and I get a phone call from CC#1's fraud division. There were 4 attempts to charge my card, so they froze it. I explained what had happened and checked CC#2 online while speaking with the representative for CC#1. Sure enough, CC#2 had 2 pending charges for the identical amount as the 4 attempts on CC#1. She advised me that the all but one of the pending charges will most likely fall off both cards, but that I should keep an eye on both cards over the next couple of days to make sure that cardcash doesn't double-dip by charging both cards for the same transaction.