Does ultimate responsibility lie in the hands of the accountant at GLBSE? If too many coins are never returned, it's not like responsibility for the shortages lies on those that got double paid.
Right? Wrong?
In a "normal" situation, the business would pay out of its own funds and then seek recovery of the over-payments by legal means (they can't just reverse them like a conventional financial institution) or write it off. In this situation, who knows whether GLBSE has the funds to cover the short-fall
or sufficient information to pursue people legally even
if they wanted to try to pursue the recovery of an equivalent fiat value through small claims (which I assume they don't want to given Nefario's fear of legal scrutiny).
If it was me, I'd probably keep the 10% and return the rest eventually but I'd let Nefario sweat for a few days first.