I think all this would never have started if deposits were correctly credited during that time period. But around that time at least three users had that issue where deposits were not "confirmed" normally by satoshidice. (Maybe because of the "malleability attack" or the bug in satoshidice that was recently corrected as mentioned in this thread).
Now I'm slightly more inclined to believe that maybe samedamnbtc's account was compromised, but if his deposit had been confirmed normally for him when he made it (in his satoshidice account), then I think he wouldn't be blaming you now.
Deposits were actually credited properly once they confirmed. Any duplicate transactions would remain as unconfirmed balance forever. So depositing 18.5 would give first 37 unconfirmed and 18.5 confirmed and 18.5 unconfirmed once one of the transactions got into a block.